You're trying to assemble one of those fiendish 10,000-piece jigsaws your aunt gives you every Christmas. There's a piece that almost fits in a critical spot, so now you're hunting through the 9000-odd remaining pieces for the one that fits perfectly.
This is the kind of problem confronting Dr Mick Foley and Dr Leann Tilley, of the Department of Biochemistry, as they work with Dr Robin Anders of the Walter and Eliza Hall Medical Research Institute to develop a malaria vaccine.
The malaria parasite Plasmodium falciparum is straight out of science fiction - a deadly shape-changer that can rapidly switch guise to deceive the immune system. But like the Death Star in "Star Wars" the seemingly indestructible parasite has a hidden weakness - somewhere on its surface is a fixture that, if it could only be identified, could become a target for the immune system's weaponry.
Dr Anders' team and others identified such an antigen several years ago - not in the malaria parasite that infects humans, but a relative, P. chabaudi, which infects mice - it is a more convenient subject for study.
P.chaubadi merozoites produce a protein called apical membrane antigen 1 (AMA-1) just before they colonise the host's red blood cells. It concentrates at the end of the parasite that contacts the host cell during the act of infection. The La Trobe-WEHI team suspects AMA-1 may be the receptor protein that the parasite uses to dock with the host cell's membrane as a prelude to infecting it.
Dr Foley says AMA-1 is a promising candidate for a malaria vaccine - disrupting its activity prevent it invading its host's red blood cells. Its structure is highly conserved between P. chabaudi and its relatives, including the human malaria parasite P.falciparum, suggesting it is crucial to the parasite's ability to complete its life cycle in its mammalian host.
Dr Anders has shown that mice injected with purified AMA-1 protein from P.chabaudii, develop a strongly protective immune response against the parasite.
In areas where malaria is endemic, children who survive infancy eventually become resistant to malaria, although they never become fully immune. They eventually exhibit antibody responses against a range of parasite antigens, including AMA-1.
But only a select few of this diverse range of antibodies are strongly protective; most make little contribution to protecting their owner. This selectivity is a consequence of the way B-cells "see" large, complex antigens like proteins.
To deal with an alien protein like AMA-1, the immune first dissects it and displays the fragments or "epitopes" to B-cells. Each B-cell line or "clone" makes a single type of antibody against a specific epitope.
So while the purified AMA-1 protein strongly protects mice against P.chabaudi, the protective response is not directed at the entire protein. It probably depends on a small subset of all B-cells that target epitopes directly involved in the binding reaction with the host cell membrane.
The same is likely to be true of the AMA-1 antigen in the human malaria parasites. So these key epitopes are ideal candidates for a potent human malaria vaccine - the vaccine would elicit an immune response more like a sniper's bullet than a shotgun blast.
Dr Foley and Dr Tilley are trying to identify these epitopes of the AMA-1 protein - not only are they candidates for a new malaria vaccine, they will help biochemists to understand just how the AMA-1 protein works, and to unravel its molecular structure.
A protein's structure is intimately linked with its function - a detailed knowledge of AMA-1's structure could lead chemists to design synthetic compounds to disrupt its function.
But all this depends on solving a molecular jigsaw puzzle: Dr Foley and Dr Tilley must identify, among a myriad antibody-secreting B-cells, the few that make antibodies to the P.chabaudi AMA-1 antigen.
Then, from this select group, they must identify the clonal lines that make antibodies against the critical epitopes of AMA-1. For this, they have turned to a powerful molecular technique called the phage antibody display system, developed in the 1989 independently by two groups in the UK and the US.
The phage antibody display system allows molecular biologists to crack the genetic code for virtually any antibody that the immune system might make against the myriad antigens its owner encounters throughout life.
B-cells are genetically "programmed" in the spleen to recognise specific antigens, so knowing that some of the mouse's B-cells recognised P.chaubaudi antigens, including AMA-1, Dr Foley and Dr Tilley isolated all the B-cells from the spleen of a mouse that had earlier been immunised against malaria by injecting it with purified AMA-1.
They then downloaded the genetic code from these B-cells into bacteriophages. Bacteriophages - or simply, "phages" are specialised viruses that infect bacteria. As they multiply and form colonies on agar plates, the infected bacteria make millions of new phage particles, each carrying a copy of the original B-cell's genetic "recipe" for its antibody.
In 1989 Dr Greg Winter's group in Cambridge and Dr Rick Lerner's group in California pioneered a technique in which the rod-like phages are genetically modified in such a way that they form with an antibody is attached to one end. Thus displayed, the antibody can review a passing parade of foreign antigens and capture any that complements its shape.
The power of the system, says Dr Foley, is that the attached phage contains the gene that gives the antibody its specificity. To identify which of the thousands of candidate phage lines recognised epitopes of the P.chaubaudii AMA-1 protein, the two La Trobe researchers coated a column with the purified protein and trickled through it a solution containing 40,000 different phage lines.
Those phages whose antibodies recognised the protein stuck to it, while the rest washed through the column.
The final yield, from 40,000 candidates, was just four antibodies that bound strongly to AMA-1. After a small modification to their genetic instructions, the phages are reintroduced into bacteria. Now, instead of ending up attached to new phage particles, the antibody molecules float free inside the bacterial cells, where it is easily extracted and purified.
Dr Foley and Dr Tilley have made further changes to the DNA-coded design of the antibodies. Normal human antibodies are Y-shaped, consisting of variable and invariant arms, attached to a stem that functions as flag, marking anything to which the antibody attaches for destruction and disposal.
The La Trobe researchers have pruned down the four antibody genes so that they only make the variable chain - the business end that determines its specificity for a particular antigen.
They plan to test each antibody against AMA-1 proteins from both the mouse and human parasites.
They are looking for an antibody that binds strongly to both, because this would confirm that the antibody is targeting a highly conserved feature, possibly shared by all Plasmodium species.
A conserved structure would indicate an indispensable function - it would be an ideal target for a designer drug to disrupt the parasite's life cycle, by locking the merozoites out of red blood cells.
From the shape of the antibody, chemists can deduce the complementary shape of the parasite antigen; a synthetic compound with a similar shape, but with an even stronger affinity for the AMA-1 antigen, could be used to disrupt its activity in vivo.
Australian researchers have already exploited this "designer drug" approach successfully in developing the potent new anti-influenza drug zanamavir.
The ultimate outcome of the La Trobe-WEHI project could be a new vaccine to prevent malaria, and a new drug to block the repeated waves of infection in people who already have malaria.
"A drug would also be very useful for tourists visiting African or South-east Asian countries where malaria is endemic," Dr Foley said. "If you're only going for a few weeks, you don't need lifetime immunity."
Given the notorious variability of malaria parasites from one region to another, the phage antibody display technology could also identify other proteins that mediate specific host-parasite reactions. This could enhance the stability and durability of malaria vaccines in different areas of the world.
"Worldwide, 200 million people are infected with malaria," Dr Foley said. "Some 2 to 3 million people die of malaria each year, making it a much bigger killer even than AIDS. Most of these deaths occur in children under the age of five."
There is no vaccine, and with resistant to traditional drugs such as quinine and chloroquine already a serious problem in many regions, the quest for new vaccines and new drugs could not be more urgent.