Mutational Analysis of Cell Cycle Inhibition by Integrin β1C

Integrin β 1C is an alternatively spliced cytoplasmic variant of the β1 subunit that potently inhibits cell cycle progression. In this study, we analyzed the requirements for growth suppression by β 1C . A chimera containing the extracellular/transmembrane domain of the Tac subunit of the human interleukin 2 receptor (gp55) fused to the cytoplasmic domain of β 1C (residues 732–805) strongly inhibited growth in mouse 10T1/2 cells even at low expression levels, whereas chimeras containing the β 1A , β 1B , β 1D , β 3 , and β 5 cytoplasmic domains had weak and variable effects. The β 1C cytoplasmic domain is composed of a membrane proximal region (732–757) common to all β 1 variants and a COOH-terminal 48-amino acid domain (758–805) unique to β 1C . The β 1C -specific domain (758–805) was sufficient to block cell growth even when expressed as a soluble cytoplasmic green fluorescent protein fusion protein. These results indicate that growth inhibition by β 1C does not require the intact receptor and can function in the absence of membrane targeting. Analysis of deletions within the β 1C -specific domain showed that the 18-amino acid sequence 775–792 is both necessary and sufficient for maximal growth inhibition, although the 13 COOH-terminal residues (793–805) also had weak activity. Finally, β 1C is known to be induced in endothelial cells in response to tumor necrosis factor and is down-regulated in prostate epithelial cells after transformation. The green fluorescent protein/β 1C (758–805) chimera blocked growth in the human endothelial cell line EV304 and in the transformed prostate epithelial cell line DU145, consistent with a role for β 1C as a growth inhibitor in vivo .