Professor Khorana has been investigating the interaction between photoactivated phosphorylated rhodopsin (P-Rh*) and arrestin as a means of understanding the molecular recognition events which mediate visual adaptation and shutoff. It has been shown that at the receptor level, rhodopsin kinase mediates the main biochemical cascade for turning off visual amplification by phosphorylation of a number of serines and threonines near the carboxy terminus of photoactivated rhodopsin. Binding of arrestin to P-Rh* then terminates the activation by preventing subsequent interaction with transducin. Prof. Khorana has expressed rhodopsin kinase from Sf21 cells and purified the three kinases that are produced; each of the forms has specific modifications at the C- and/or N-termini including isoprenyl and myristoyl groups. The goal of this research is to understand the molecular recognition events that mediate visual adaptation and shutoff using site-directed mutagenesis. A particular focus of the collaboration with the resource involves the characterization of three different forms of rhodopsin kinase, an enzyme key to the shutoff regulation of phototransduction. The different forms are believed to be differentially posttranslationally modified by isoprenylation, carboxymethylation, and myristoylation. It is proposed to determine the MWs of the different forms of rhodopsin kinase and then to elucidate the types and locations of the postranslational modifications. Currently, MALDI-TOF MS and ESI-MS are being utilized to determine the molecular weights of the intact proteins. In future studies, the types and locations of posttranslational modifications will be elucidated.
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