Stimulation of cleavage of membrane proteins by calmodulin inhibitors

The ectodomain of several membrane-bound proteins can be shed by proteolyti c cleavage. The activity of the proteases involved in shedding is highly re gulated by several intracellular second messenger pathways, such as protein kinase C (PKC) and intracellular Ca2+. Recently, the shedding of the adhes ion molecule L-selectin has been shown to be regulated by the interaction o f calmodulin (CaM) with the cytosolic tail of L-selectin. Prevention of CaM -L-selectin interaction by CaM inhibitors or mutation of a CaM binding site in L-selectin induced L-selectin ectodomain shedding. Whether this action of CaM inhibitors also affects other membrane-bound proteins is not known. In the present paper we show that CaM inhibitors also stimulate the cleavag e of several other transmembrane proteins, such as the membrane-bound growt h factor precursors pro-transforming growth factor-alpha and pro-neuregulin -alpha 2c, the receptor tyrosine kinase, TrkA, and the beta-amyloid precurs or protein. Cleavage induced by CaM inhibitors was a rapid event, and resul ted from the activation of a mechanism that was independent of PKC or intra cellular Ca2+ increases, but was highly sensitive to hydroxamic acid-based metalloprotease inhibitors. Mutational analysis of the intracellular domain of the TrkA receptor indicated that CaM inhibitors may stimulate membrane- protein ectodomain cleavage by mechanisms independent of CaM-substrate inte raction.