Carboxyl methylation and farnesylation of transducin gamma-subunit synergistically enhance its coupling with metarhodopsin II.

Abstract

A heterotrimeric G‐protein in vertebrate photoreceptor cells is called transducin (T alpha beta gamma), whose gamma‐subunit is a mixture of two components, T gamma‐1 and T gamma‐2. T gamma‐2 is S‐farnesylated and partly carboxyl methylated at the C‐terminal cysteine residue, whereas T gamma‐1 lacks the modified cysteine residue. To elucidate the physiological significance of the double modifications in T gamma, we established a simple chromatographic procedure to isolate T gamma‐1, methylated T gamma‐2 and non‐methylated T gamma‐2 on a reversed phase column. Taking advantage of the high and reproducible yield of T gamma from the column, we analyzed the composition of T gamma subspecies in the T alpha‐T beta gamma complex which did not bind with transducin‐depleted rod outer segment membranes containing metarhodopsin II. The binding of T alpha‐T beta gamma with the membranes was shown to require the S‐farnesylated cysteine residue of T gamma, whose methylation further enhanced the binding. This synergistic effect was not evident when T alpha was either absent or converted to the GTP‐bound form which is known to dissociate from T beta gamma. Thus we concluded that a formation of the ternary complex, T alpha‐T beta gamma‐metarhodopsin II, is enhanced by the farnesylation and methylation of T gamma. This suggests that the double modifications provide most efficient signal transduction in photoreceptor cells.

Stimulation of phospholipase C‐β2 by recombinant guanine‐nucleotide‐binding protein βγ dimers produced in a baculovirus/insect cell expression system

“…proteolytic removal of the carboxy-terminal tripeptide and/or carboxy methylation, are the critical determinants of the P y dimerlPLCP, interaction. Of interest, carboxy methylation and farnesylation of yl have been suggested to synergistically enhance the coupling of retinal transducin to metarhodopsin I1 [37]. Likewise, both isoprenylation and carboxy methylation are critical for the biologic activity of fungal mating factors .…”

A new type of mitogenic factor produced by Streptococcus pyogenes

“…'tcefic acid containing S0~ acetonitrilo and infused into the ion source ~1 a flow rate of 20 /JI/min. Details of the procedure were described previously [12],…”

Identification and isolation of common and tissue‐specific geranylgeranylated γ subunits of guanine‐nucleotide‐binding regulatory proteins in various tissues

“…Subsequent removal of the three terminal residues (Aaa-Aaa-Xaa) leads to the methyl-esterification of newly exposed a-carboxy group of the cysteine [26, 301. These modifications are necessary for the association of G proteins with their receptors [31] or with membranes [32 -341 that is required to elicit biological activity. When we consider the physiological significance of the modifications, it is interesting to ask whether the difference in chain length of the isoprenyl group (farnesyl or geranylgeranyl) depends on tissues or functions of G proteins.…”