TM4SF1: a tetraspanin-like protein necessary for nanopodia formation and endothelial cell migration.

Transmembrane-4-L-six-family-1 (TM4SF1) is a tetraspanin-like membrane protein that is highly and selectively expressed by cultured endothelial cells (EC) and, in vivo, by EC lining angiogenic tumor blood vessels. TM4SF1 is necessary for the formation of unusually long (up to a 50 μm), thin (~100-300 nm wide), F-actin-poor EC cell projections that we term 'nanopodia'. Immunostaining of nanopodia at both the light and electron microsopic levels localized TM4SF1 in a regularly spaced, banded pattern, forming TM4FS1-enriched domains.