Secreted protein kinases

Abstract

Protein kinases constitute one of the largest gene families and control many aspects of cellular life. In retrospect, the first indication for their existence was reported 130 years ago when the secreted protein, casein, was shown to contain phosphate. Despite its identification as the first phosphoprotein, the responsible kinase has remained obscure. This conundrum was solved with the discovery of a novel family of atypical protein kinases that are secreted and appear to phosphorylate numerous extracellular proteins, including casein. Fam20C, the archetypical member, phosphorylates secreted proteins within Ser-x-Glu/pSer motifs. This discovery has solved a 130-year-old mystery and has shed light on several human disorders of biomineralization.

Figure I. Mechanism for the phosphorylation of extracellular proteins by secreted kinases

Proteins destined for secretion have a short sequence of hydrophobic amino acids at the N-terminus known as the signal peptide (SP, shaded black). During translation, the SPs of the kinase (red) and substrate (blue) bind the signal recognition particle embedded in the endoplasmic reticulum (ER) membrane and the nascent polypeptides enter the lumen of the ER. The signal peptidase subsequently cleaves the SP releasing the proteins from the membrane. ATP and divalent cations (M²⁺) are transported into the Golgi lumen by specific membrane bound transporters. Catalysis may occur within the Golgi lumen or in the extracellular space.

Figure 1. Four-jointed family of protein kinases

(A) The architecture of a secreted protein kinase consists of an N-terminal SP, a C-terminal kinase domain and no transmembrane domain. (B) A phylogenetic tree depicting the human four-jointed family of protein kinases (Fjx1, Fam20A, Fam20B, Fam20C, Fam198A and Fam198B). The bacterial kinase, HipA shows weak sequence similarity to the four-jointed family of protein kinases.

Figure 2. Fam20B phosphorylates xylose within the glycosaminoglycan-protein linkage region

Glycosaminoglycans are synthesized by the attachment of a tetrasaccharide linkage to specific Ser residues within core proteins during transit through the secretory pathway. The tetrasaccharide consists of the monosaccharides xylose (Xyl), galactose (Gal), and glucuronic acid (GlcA) linked by glucosidic bonds in a GlcAβ1-3Galβ1-3Galβ1-4Xylβ1 orientation. Fam20B transfers a phosphate from ATP to the 2′-OH of xylose (β-Xyl), which may regulate heparin and chondroitin sulfate formation.

Figure 3. The secretory calcium binding phosphoprotein (SCPP) family

Shown is the small region on human chromosome 4 in which 23 of the 24 SCPP family members are located. Amelogenin X is on the X chromosome. The SCPPs are secreted phosphoproteins (phosphorylated on multiple S-x-E/S motifs), have a high affinity for calcium ions and regulate biomineralization. The Pro/Gln rich SCPPs and the acidic SIBLING SCPPs are shown in blue and red, respectively Adapted from [60].

Figure 4. Relevant contribution of Fam20C to the generation of the human phosphoproteome

Data have been extrapolated from 35679 Ser/Thr phosphosites available in the PhosphoSitePlus by performing a two sample logo (“ice-logo”) analysis [70] for each kinase as described by Salvi et al. [71]. In the case of Fam20C, only phosphosites conforming to the consensus S-x-E/pS but not to that of CK2 (S/T-x-x-E/D/pS) have been considered.