A proline-rich region and nearby cysteine residues target XL alpha s to the Golgi complex region

XL alpha s is a splice variant of the heterotrimeric G protein, G alpha(s), found on Golgi membranes in cells with regulated and constitutive secretio n. We examined the role of the alternatively spliced amino terminus of XL a lpha s for Golgi targeting with the use of subcellular fractionation and fl uorescence microscopy. XL alpha s incorporated [H-3]palmitate, and mutation of cysteines in a cysteine-rich region inhibited this incorporation and le ssened membrane attachment. Deletion of a proline-rich region abolished Gol gi localization of XL alpha s 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 prolin e-rich regions, including a G alpha(i2) splice variant, dynamin II, beta II I spectrin, comitin, and a Golgi SNARE, GS32. Our results suggest that prol ine-rich regions can be a Golgi-targeting signal for G protein alpha subuni ts and possibly for other peripheral membrane proteins as well.