A Proline-rich Region and Nearby Cysteine Residues Target XLαs to the Golgi Complex Region

Abstract

XLαs is a splice variant of the heterotrimeric G protein, Gαs, found on Golgi membranes in cells with regulated and constitutive secretion. We examined the role of the alternatively spliced amino terminus of XLαs for Golgi targeting with the use of subcellular fractionation and fluorescence microscopy. XLαs incorporated [3H]palmitate, and mutation of cysteines in a cysteine-rich region inhibited this incorporation and lessened membrane attachment. Deletion of a proline-rich region abolished Golgi localization of XLαs without changing its membrane attachment. The proline-rich and cysteine-rich regions together were sufficient to target the green fluorescent protein, a cytosolic protein, to Golgi membranes. The membrane attachment and Golgi targeting of the fusion protein required the putative palmitoylation sites, the cysteine residues in the cysteine-rich region. Several peripheral membrane proteins found at the Golgi have proline-rich regions, including a Gαi2 splice variant, dynamin II, βIII spectrin, comitin, and a Golgi SNARE, GS32. Our results suggest that proline-rich regions can be a Golgi-targeting signal for G protein α subunits and possibly for other peripheral membrane proteins as well.