The mammalian calpain gene family curently contains 13 distinct large subunit products most of which complex with one of two smaller 30kDa subunits. ( An excellent introduction to the Calpain family can be found on a web site created by Valery Thompson http://ag.arizona.edu/calpains/index.html ) Spreading cells display newly transiently formed integrin adhesion clusters containing Calpain1 (mu-Calpain), cleaved Talin, B3-Integrin and SPTAN1(Spectrin). Recruitment of Rac to these clusters leads to the activation of Rac and the formation of Rac-induced Focal clusters. The Calpain1 in the integrin clusters initially inactivates RhoA allowing for the formation of lamellipodia. The subsequent activation of newly synthesized RhoA transforms these clusters into Focal Adhesion Complexes and the formation of contractile actin-myosin stress fibers. These mature adhesions do not contain calpain.
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