Preferential Assembly of G- Complexes Directed by the g Subunits - GPCR Signalling Complexes: Synthesis, Assembly, Trafficking and Specificity - page 175

10.1 Introduction

The mammalian genome contains nearly 1,000 seven-transmembrane receptors that
signal through heterotrimeric G proteins to regulate downstream effectors
(Fredriksson et al. 2003 ; Lagerström and Schiöth 2008 ). The assembly of the G-a b g
complex is a key step in this process. In particular, combinatorial association of the
known number of 16 a, 5 b, and 12 g subunit genes provides the potential to create
more than 1,000 distinct G-a b g complexes (Hildebrandt 1997 ; Hurowitz et al.
2000 ). Although certain interactions are precluded, biochemical studies show that
most G-a b g associations are physically possible (Richardson and Robishaw 1999 ;
Jeong and Ikeda 2000 ). Most likely, this reflects the high homology between related
members of each a , b , g subunit class that allow them to substitute for one and
another in an artificial system where physical organization of these signaling path-
ways has been disrupted (Neubig 1994 ). More recently, reverse genetic studies have
begun to identify the specific G-a b g combinations that are assembled in different
physiological contexts (Robishaw 2009 ; Schwindinger et al. 2003, 2004, 2009,
2010 ; Lobanova et al. 2008 ). Presumably, the generation of specific G-a b g combi-
nations depends upon cellular factors that contribute to the assembly process, and
highlights a large gap in our knowledge regarding which G-a b g complexes actually
exist, and what instructional cues control their selective assembly. In this chapter,
we will focus on new results indicating preferential assembly of the striatal-specific
G olf protein occurs by a sequential process that is directed by the g 7 subunit.

10.2 Do Preferential G- a b g Combinations Exist?

Deciphering which G-a b g combinations actually exist in an intact system has been
a challenge. In sensory cells, transcription is an important mechanism for limiting
the number of possible G-a b g combinations as revealed by co-localization strate-
gies. For instance, retinal rod cells express a specific G-a t1 b 1 g 1 heterotrimer that is
necessary for night vision, whereas retinal cone cells express a different G-a t2 b 3 g 8
combination that is required for color vision (Peng et al. 1992 ) . In an analogous
fashion, olfactory neurons express a unique G- a olf b 1 g 13 heterotrimer that is neces-
sary for smell (Kerr et al. 2008 ), whereas gustatory cells express a novel G-a t3 b 1 g 13
combination that is required for taste (Huang et al. 1999 ) .
Despite a role for transcription in driving the preferential assembly of G-a b g
associations in highly specialized cells, it is clear that most other cell types must
express a broad repertoire of G-a b g combinations consistent with their greater
diversity of functions. Because detergent solubilization of G-a b g trimers speeds up
their innate tendency to dissociate into a and b g subunits, co-immunopreciptation
and co-purification approaches have not proven very useful in identifying which
G-a b g complexes actually exist (Wilcox et al. 1995 ) . However, reverse genetics
strategies have begun to provide this information (Robishaw 2009 ; Robishaw and
Berlot 2004 ). Because synthesis of G-a b g trimers occurs in the cytosol (Rehm and
Ploegh 1997 ) and subsequent membrane translocation is dependent in part upon