The MADS box is a highly conserved sequence motif found in a family of transcription factors. The conserved domain was recognized after the first four members of the family, which were MCM1, AGAMOUS, DEFICIENS and SRF (serum response factor). The name MADS was constructed form the "initials" of these four "founders".
By now, more than hundred MADS box sequences have been found in species from all eukaryotic kingdoms. The family of MADS domain proteins has been subdivided into several distinct subfamilies. Most MADS domain factors play important roles in developmental processes. Most prominently, the MADS box genes in flowering plants are the "molecular architects" of flower morphogenesis.
All MADS-domain proteins known to date have additional domains attached to the C-terminus of the MADS-domain, and some also have N-terminal extensions. According to this overall structure, MADS-domain proteins can be subdivided into three different types, the MEF2 type, the ARG80 type and the plant type MADS-domain proteins. These three structural types are shown in the following sketch that shows an example of each type:
0 100 200 300 400
+-------------------+-------------------+-------------------+-------------------+
+--+-----------+------+------------------------------+
MCM1: | | MADS | SAM | |
+--+-----------+------+------------------------------+
+-----------+------+------------------------------------------------------+
MEF2B: | MADS | MEF2 | |
+-----------+------+------------------------------------------------------+
+-----------+-------+-----------+--------------+
DEF: | MADS | I | K | C |
+-----------+-------+-----------+--------------+
|<- MADS -->|
domain
|<-------- MIK domain --------->|
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