Constitutive Cleavage of the Single-Pass Transmembrane Protein Alcadein[alpha] Prevents Aberrant Peripheral Retention of Kinesin-1

Various membrane proteins are shed by proteinases, constitutively and/or when stimulated by external signals. While the physiological significance of external signal-induced cleavages has been intensely investigated, relatively little is known about the function of constitutive cleavages. Alcadein[alpha] (Alc[alpha]; also called Calsyntenin-1) is an evolutionarily conserved type I single-pass transmembrane protein that binds to kinesin-1 light chain (KLC) to activate kinesin-1's transport of Alc[alpha]-containing vesicles. We found that Alc[alpha] was constitutively and efficiently cleaved to liberate its ectodomain into the extracellular space, and that full-length Alc[alpha] protein was rarely detected on the cell surface. The secretion efficiency of the ectodomain was unaltered by a mutation that both abolished Alc[alpha]'s KLC-binding activity and attenuated its peripheral transport, suggesting that Alc[alpha]'s cleavage occurred, at least partly, en route to the cell surface. We further demonstrated that uncleavable mutant Alc[alpha] proteins readily accumulated on the cell surface and induced aberrant peripheral recruitment of KLC1 and kinesin heavy chain. Our observations suggest that Alc[alpha] is efficiently processed in part to minimize the inappropriate peripheral retention of kinesin-1. This role might exemplify the functional relevance of the constitutive cleavage of single-pass transmembrane proteins.