This chapter highlights small G proteins. It also reviews their possible functions and modes of action in comparison with those of ras p21. Three groups of G proteins have both GDP/GTP-binding and GTPase activities. The first group of G proteins includes G proteins involved in protein synthesis. The second group of G proteins comprises a superfamily of the GsGiGoGqand transducin families. G proteins of the third group are monomeric and show molecular weight values between 20,000 and 36,000. These G proteins are called “small G proteins” and are composed of more than 40 members. Small G proteins play important roles in regulating various cell functions. The G proteins involved in protein synthesis and the heterotrimeric G proteins serving as transducers for membrane receptors have two interconvertible forms, GDP-bound inactive and GTP-bound active forms. The GDP-bound inactive form is converted to the GTP-bound active form by a GDP/GTP exchange reaction. All of the small G proteins have the consensus amino acid sequences responsible for GDP/GTP-binding and GTPase activities. Small G proteins also possess unique C-terminal sequences.
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