beta 1B integrin subunit contains a double lysine motif that can cause accumulation within the endoplasmic reticulum

Human epidermal keratinocytes are one of the few cell types that express th e beta 1B splice variant of the pi integrin subunit. Although in transfecti on experiments beta 1B acts as a dominant negative inhibitor of cell adhesi on, we found that beta 1B was expressed at very low levels in keratinocytes , both in vivo and in culture, and had a predominantly cytoplasmic distribu tion, concentrated within the endoplasmic reticulum. To examine why beta 1B accumulated in the cytoplasm, we prepared chimeras between CD8 alpha and t he beta 1A and alpha 1B integrin cytoplasmic domains. In transfected HeLa c ells, both constructs reached the cell surface but the rate of maturation o f the beta 1B chimera was considerably retarded relative to beta 1A. The be ta 1B cytoplasmic domain contains two lysine residues that resemble the dou ble lysine motif characteristic of many proteins that are resident within t he endoplasmic reticulum. Mutation of each lysine individually to serine ha d no effect on CD8 beta 1B maturation, but when both residues were mutated the rate of CD8 beta 1B maturation increased to that of CD8 beta 1A. Furthe r analysis of beta 1B function in keratinocytes must, therefore, take into account the low abundance of the isoform relative to beta 1A and the potent ial for beta 1B to accumulate in the endoplasmic reticulum. (C) 2000 Wiley- Liss, Inc.