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Figure 3.
FIGURE 3. The phosphorylation sites of GRK1. a, the
RH-kinase core of GRK1. The structure corresponds to that of
crystal form I (with composite C-terminal extension; see Fig.
2). The Ser^5, Thr^8, Ser^21, Ser^488, and Thr^489
phosphorylation sites are drawn as stick models. The expected
position of the membrane plane is indicated. Top inset, the
Ser^488 and Thr^489 phosphorylation sites correspond to the AGC
kinase turn motif. Bottom inset, interaction of Thr(P)^8 with
the RH domain. Gln^73 and Glu^93 form direct hydrogen bonds,
whereas Lys^69 and Lys^90 complement the charge of the phosphate
moiety. These crystals grew at pH 4.35, and so either Glu^93 or
the phosphate group could be protonated. b, tandem mass
spectrometry spectra of phosphopeptides from GRK1[535]-His[6]
(Pool A, pretreated with 4 mM ATP and 2 mM MgCl[2]). Both Ser^5
and Thr^8 sites were identified in a single peptide. The Ser^5
site was also readily observed in endogenous GRK1, as were the
previously observed phosphorylation sites at Ser^21, Ser^488,
and Thr^489 (supplemental Fig. S7).
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