IOVS Journal of Bacteriology
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(Investigative Ophthalmology and Visual Science. 2003;44:4622-4629.)
© 2003 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.03-0420
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Cone Photoreceptor ß{gamma}-Transducin: Posttranslational Modification and Interaction with Phosducin

Fayu Chen,1 Pei-San Ng,2 Kym F. Faull,3 and Rehwa H. Lee1,2

1From the Jules Stein Eye Institute, UCLA School of Medicine, Los Angeles, California; the 2Molecular Neurology Laboratory, VA Greater Los Angeles Healthcare System at Sepulveda, Sepulveda, California; the 3Pasaraw Mass Spectrometry Laboratory, Department of Chemistry, Biochemistry, Psychiatry and Behavioral Science, and The Neuropsychiatry Institute, UCLA, Los Angeles, California.

PURPOSE. To characterize the structure of cone ß{gamma}-transducin (Tß3{gamma}8) and its interaction with phosducin (pdc).

METHODS. The T{gamma}8 subunit of Tß3{gamma}8 was isolated by column chromatography for peptide mapping with mass spectrometry. Tß3{gamma}8 was compared with rod ß{gamma}-transducin (Tß1{gamma}1) in terms of the electrophoretic mobility, pdc binding affinity, and the effects of phosphorylation and methylation, and then the correlation to the crystal structures and functional domains of Tß1{gamma}1 was determined.

RESULTS. The mature T{gamma}8 is a 65-amino-acid peptide encoded by the G{gamma}8 gene with an acetylated and a farnesylated–methylated N- and C-terminus, respectively. Purified Tß3{gamma}8 is similar to Tß1{gamma}1 in that (1) both are heterogeneous, containing methylated and demethylated T{gamma} subunits; (2) each demethylated dimer migrates faster than its methylated counterpart during native gel electrophoresis, and the methylation-associated mobility differential is masked by pdc binding; and (3) both dimers bind pdc with the same affinity, and the affinity is reduced threefold by PKA phosphorylation of pdc and twofold by demethylation at the C-terminus of T{gamma}. Tß3{gamma}8 differs from Tß1{gamma}1 in exhibiting lower intrinsic electrophoretic mobility, and the difference is unaffected by either pdc binding or the status of T{gamma} methylation.

CONCLUSIONS. Tß3{gamma}8 is identical with Tß1{gamma}1 in T{gamma} isoprenylation, the spatial organization, and the mode of pdc binding, indicating that its interaction with pdc does not play an important role in the specialization of cones. Changes in Tß{gamma} characteristics by T{gamma} methylation reveal conformational changes on a surface domain that is essential for Tß{gamma} functions and support a regulatory role for reversible methylation.






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Copyright © 2003 by the Association for Research in Vision and Ophthalmology