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Subunits
of Heterotrimeric G Proteins
Center for Cancer Research, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
The assembly in living cells of heterotrimeric
guanine nucleotide binding proteins from their constituent 
, 
, and 
subunits is a complex process, compounded by the multiplicity of the genes that encode
them, and the diversity of receptors and effectors with
which they interact. Monoclonal anti- antibodies
(ARC5 and ARC9), raised against immunoaffinity purified complexes, recognize subunits when not associated with and can thus be used to monitor assembly of complexes. Complex formation starts
immediately after synthesis and is complete within 30 min. Assembly occurs predominantly in the cytosol,
and association of complexes with the plasma membrane fraction starts between 15-30 min of chase. Three
pools of subunits can be distinguished based on their
association with subunits, their localization, and their detergent solubility. Association of and subunits
with detergent-insoluble domains occurs within 1 min
of chase, and increases to reach a plateau of near complete detergent resistance within 30 min of chase.
Brefeldin A treatment does not interfere with delivery of subunits to detergent-insoluble domains, suggesting that assembly of G protein subunits with their receptors occurs distally from the BFA-imposed block of
intracellular membrane trafficking and may occur directly at the plasma membrane.
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