XL alpha s is a new type of G protein

The GTP-binding proteins are well-known regulators of cellular functions, including vesicular transport. Cholera toxin, which is known to catalyse ADP-ribosylation of the alpha s subunit of heterotrimeric G proteins, stimulates secretory vesicle formation from the trans-Golgi network. Here we describe a new cholera toxin target, an 'extra large' G protein (XL alpha s; M(r) 92K) which consists of a new 51K XL-portion linked to a G alpha s truncated at the amino terminus. XL alpha s is specifically associated with the trans-Golgi network and occurs selectively in cells containing both the regulated and the constitutive pathway of protein secretion. Hence, XL alpha s may mediate the effects of cholera toxin on secretory vesicle formation.