use PRATT or advPRATT to generate pattern
use ScanProsite or 3motif to scan that pattern in Swiss-Prot, PDB, etc.

[EQ]-x-L-Y-[DQSTE]-x(3,12)-[LIV]-[ST]-Y-x-R-[ST]-[DEQS]
[eq].ly[dqste]{0,1}.{3,12}[liv][st]y.r[st][deqs]