Enzymatic and immunological detection of a G‐protein inHalobacterium halobium

In membrane preparations ofHalobacterium, the hydrolysis of cGMP is accelerated by activators of G‐proteins, namely GTP, GTP‐γ‐S, and fluoroaluminate. This suggests a type of G‐protein which acts on a phosphodiesterase. Light stimuli which evoke behavioral responses in intact bacteria influence the rate of cGMP hydrolysis. Using an antiserum raised against a peptide identical with one of the sequences presumably involved in GTP binding of most G‐proteins, a cross reactive protein with an apparent molecular mass of 59 kDa could be detected on immunoblots. The results support the idea that a G‐protein may be part of the photosensory transduction chain ofHalobacterium[(1987) Biochim. Biophys. Acta 923, 222–232].