Intracellular Posttranslational Modifications

The intracellular domains, that is, the intracellular loop i1 - 3 as well as the C-terminus of FZDs, are putative sites for both protein-protein interaction [9] and posttranslational modifications (see Table 5.1 ; Figs. 5.1 and 5.2). Even though all FZDs contain plenty of putative phosphorylation sites—mainly for serine/threonine kinases but also some tyrosine kinases (as determined by the MiniMotif Miner software [32])—only one study actually reports DVL-dependent phosphorylation of XFZD3 ; at serine 576 [33]. Phosphorylation at this site and other unidentified sites in the XFZD3 C-terminus play a role in downregulation of XFZD3 signaling. Analysis of the C-terminal sequence of XFZD3 with the MiniMotif Miner suggests a casein kinase 1 site at serine 576 that requires a phosphorylated serine in position 573. Ser573 resembles a 90kDa ribosomal S6 kinase (RSK)-phosphorylation site motif.

As shown in Table 5.1, all human FZDs are predicted to have many putative phosphorylation sites, which suggests a high degree of modulation through possible WNT-induced feedback, for example, through CK1/2 (casein kinase 1 and 2), GSK3 (glycogen synthase kinase 3), CamKII (Ca2+- and calmodulin-dependent kinase II), and PKC (Ca2+-dependent protein kinase). In addition, there may also be cross- t alk with other signaling pathways, such as various kinases involved in cell cycle regulation, for example, polo-like kinase (PLK), p70S6 kinase, p90 S6 ribosomal kinase (RSK), ataxia telangiectasia mutated (ATM) kinase, cyclic AMP-dependent protein kinase (PKA), phosphorylase kinase, EGF (epidermal growth factor) receptor, and Janus kinase (JAK) signaling. Prediction of the putative phosphorylation sites is based on published consensus sites for kinases. The list remains to be confirmed, but it is far from complete and leaves out G protein-coupled receptor kinases (GRKs), which are known to regulate GPCR phosphorylation, thus altering their dynamics and activity [34].

Of special interest are the putative phosphorylation sites that appear frequently not only in the DVL-binding sequence KTxxxW, but also those in the terminal PDZ ligand domain. Phosphorylation in either of these regions might have an effect on protein-protein interaction between FZD and DVL, or between FZD and other proteins. Thus, it will be exciting to watch future developments as we learn more about the roles of various kinases, FZD phos-phorylation sites, and possible cross-talk for WNT-induced receptor signaling, dynamics, and desensitization [35].

TABLE 5.1 Summary of Kinases (according to MiniMotif Miner) with Putative Phosphorylation Sites in the Intracellular Domains of FZD1-10

Human FZDs

PKA

PKC

PKG

CK1

CK2

CamKII

RSK

p70 S6 kinase

1/2

ATM kinase

FZDjJl FZDj_i2 FZD1_i3 FZD1_ c-term

V

V (x2)

V

V

V

V(x2)

V

FZD2_i1 FZD2_i2 FZD2_i3 FZD2_ c - term

V

V(x2)

V

V

V

V

V

FZD3_i1 FZD3_i2 FZD3_i3

fzd3_

c - term

V(x6)

V (x4)

V (x 7)

V(x2)

V (x 5)

V (x 6)

V

V

V (x2)

FZD4_i1 FZD4_i2 FZD4_i3

fzd4_

c - term

V

V(x2)

V

V

V(x2)

FZD5_i1 FZD5_i2 FZD5_i3

fzd5_

c - term

V (x2)

V (x 4)

V

V

V

V(x2)

V(x3)

FZD6_i1 FZD6_i2 FZD6 i3

fzd6_

c - term

V (x2)

V (x8)

V

V (x 8)

V (x 7)

V

V(x5)

V

FZD7_i1 FZD7_i2 FZD7_i3

fzd7_

V

V(x3)

V

V(x2)

V

V

FZDs_i1 FZDs_i2 FZDs_i3 FZD«_ c - term

V(x3)

V(x6)

V

V

FZD9_i1 FZD9_i2 FZD9_i3

fzd9_

c - term

V

V

V

V

V

JAK2 GSK- GSK- PLK CDK Phosphorylase ABL EGFR TPK-IIB/ CSK 3alpha 3beta kinase p38Syk

TABLE 5.1 (Continued)

Human FZDs

PKA

PKC

PKG CK1 CK2

CamKII RSK

p70 S6 kinase

1/2

ATM kinase

FZD10_i1 FZD10_i2 FZD10_i3 FZDio_ c-term

V

V(x3)

V V

V (x2)

(i1: intracellular loop 1; i2: intracellular loop 2; i3: intracellular loop 3; c-term: C-terminus). V indicates a single, whereas numbers in parentheses indicate multiple phosphorylation sites. Kinases: PKA, cyclic AMP-dependent protein kinase; PKC, Ca2+-dependent protein kinase; PKG, cyclic GMP-dependent protein kinase; CK1, casein kinase 1; CK2, casein kinase 2; CamKII, Ca2+- and calmodulin-dependent protein kinase; RSK, 90kDa ribosomal S6 kinases; ERK1/2, extracellular signalregulated protein kinase1/2; ATM kinase, Ataxia telangiectasia-mutated kinase; JAK2, Janus kinase 2 (autophosphorylation site); GSK3a/p, glycogen synthase kinase 3a/P; CDK, cyclin-dependent kinases; ABL, product of the c-abl oncogene; EGFR, epidermal growth factor receptor; tyrosine protein kinase TPK-IIB/p38Syk; CSK, carboxy-terminal Src kinase.

human FZD, human FZD,

(GSK3a)

^ FZD phosphorylation could affect protein recruitment, relocalization/internalization WNT binding, and desensitization

Figure 5.2 Schematic view of human FZD4 containing putative phosphorylation sites on intracellular domains. For abbreviations, see Table 5.1.

0 0

Post a comment