PEST MOTIFS ARE NOT REQUIRED FOR RAPID CALPAIN-MEDIATED PROTEOLYSIS OF C-FOS PROTEIN

Cytoplasmic degradation of c-fos protein is extremely rapid. Under cer tain conditions, it is a multi-step process initiated by calcium-depen dent and ATP-independent proteases called calpains. PEST motifs are pe ptide regions rich in proline, glutamic acid/aspartic acid and serine/ threonine residues, commonly assumed to constitute built-in signals fo r rapid recognition by intracellular proteases and particularly by cal pains. Using a cell-free degradation assay and site-directed mutagenes is, we report here that the three PEST motifs of c-fos are not require d for rapid cleavage by calpains. Testing the susceptibility of PEST m otif-bearing and non-bearing transcription factors including GATA1, GA TA3, Myo D, c-erbA, Tal-1 and Sry, demonstrates that PEST sequences ar e neither necessary nor sufficient for specifying degradation of other proteins by calpains. This conclusion is strengthened by the observat ion that certain proteins, reportedly known to be cleavable by calpain s, are devoid of PEST motifs.