Abstract
The chloroplast signal recognition particle consists of a conserved
54-kDa GTPase and a novel 43-kDa chromodomain protein (cpSRP43) that
together bind light-harvesting chlorophyll a/b-binding protein (LHCP)
to form a soluble targeting complex that is subsequently directed
to the thylakoid membrane. Homology-based modeling of cpSRP43 indicates
the presence of two previously identified chromodomains along with
a third N-terminal chromodomain. Chromodomain deletion constructs
were used to examine the role of each chromodomain in mediating distinct
steps in the LHCP localization mechanism. The C-terminal chromodomain
is completely dispensable for LHCP targeting/integration in vitro.
The central chromodomain is essential for both targeting complex
formation and integration because of its role in binding the M domain
of cpSRP54. The N-terminal chromodomain (CD1) is unnecessary for
targeting complex formation but is required for integration. This
correlates with the ability of CD1 along with the ankyrin repeat
region of cpSRP43 to regulate the GTPase cycle of the cpSRP-receptor
complex.
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