Carboxyl methylation and farnesylation of transducin gamma-subunit synergistically enhance its coupling with metarhodopsin II.
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.
Ohguro
H
H
Department of Ophthalmology, Sapporo Medical College, Japan.
Fukada
Y
Y
Takao
T
T
Shimonishi
Y
Y
Yoshizawa
T
T
Akino
T
T
D000595:Amino Acid Sequence; D000818:Animals; D002264:Carboxylic Acids / Q000378:metabolism; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D004591:Electrophoresis, Polyacrylamide Gel; D005204:Farnesol / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism; D008745:Methylation; D008969:Molecular Sequence Data; D012243:Rhodopsin / Q000031:analogs & derivatives* / Q000378:metabolism; D016339:Spectrometry, Mass, Fast Atom Bombardment; D015293:Transducin / Q000378:metabolism*
D002264:Carboxylic Acids; D005204:Farnesol; C013110:metarhodopsins; D012243:Rhodopsin; D019204:GTP-Binding Proteins; D015293:Transducin