Golgi serine/threonine protein kinase that phosphorylates secretory pathway proteins within Ser-x-Glu/pSer motifs and plays a key role in biomineralization of bones and teeth (PubMed:22900076, PubMed:22732358, PubMed:25789606). Constitutes the main protein kinase for extracellular proteins, generating the majority of the extracellular phosphoproteome (By similarity). Mainly phosphorylates proteins within the Ser-x-Glu/pSer motif, but also displays a broader substrate specificity (By similarity). Phosphorylates ERO1A, enhancing its activity which is required to maintain endoplasmic reticulum redox homeostasis and for oxidative protein folding (By similarity). During endoplasmic reticulum stress, phosphorylates P4HB/PDIA1 which induces a functional switch, causing P4HB to change from an oxidoreductase to a molecular chaperone (By similarity). This is critical to maintain ER proteostasis and reduce cell death under ER stress (By similarity). Phosphorylation of P4HB also promotes its interaction with ERN1, leading to reduced activity of ERN1, a key sensor for the endoplasmic reticulum unfolded protein response (By similarity). Required for osteoblast differentiation and mineralization (By similarity). Phosphorylates casein as well as a number of proteins involved in biomineralization such as AMELX, AMTN, ENAM and SPP1 (PubMed:25789606). In addition to its role in biomineralization, also plays a role in lipid homeostasis, wound healing and cell migration and adhesion (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Serine/threonine protein kinase activity is increased upon interaction with FAM20A. kcat is 52 min(-1). Homodimer; disulfide-linked (By similarity). Interacts with FAM20A; probably forming a heterotetramer of 2 subunits of FAM20A and 2 subunits of FAM20C (PubMed:25789606). Interacts with COPII components SEC23A and SEC24A; transport of FAM20C from the endoplasmic reticulum to the Golgi is likely to be mediated by COPII vesicles (By similarity). Resides in the Golgi apparatus membrane and is secreted following propeptide cleavage. Retained in the endoplasmic reticulum (ER) in response to ER stress where it phosphorylates P4HB. In the mammary gland, expressed at higher levels in lactating mice than in virgin mice (at protein level) (PubMed:29858230). Highly expressed in the tooth. No expression in the dental pulp. At the secretory stage of amelogenesis, it is detected in the matrix of the enamel, in the ameloblasts, and within the cells adjoining the stratum intermedium (a tissue layer analogous to the stellate reticulum seen in the developing molar). Strong expression is observed in maturation stage ameloblasts and throughout the non-cornified layers of the gingival epithelium. Expressed at moderate levels in bone and at low levels in kidney, liver, brain and lung. Very low expression, if any, in spleen and skeletal muscle. In the developing tooth, initial expression observed in odontoblasts at all stages of development. At later stages, restricted expression pattern in ameloblasts. Also observed in osteoblasts in the alveolar bone. N-glycosylation is required for folding. Autophosphorylated. Propeptide cleavage by MBTPS1/S1P promotes FAM20C secretion and maximal kinase activity which is essential for efficient osteoblast differentiation and biomineralization. Mice survive to adulthood and show biomineralization defects, suc as severe amelogenesis imperfecta (AI). In addition, mice are severely hypophosphatemic and develop notable lesions in both dentin and bone. Belongs to the FAM20 family. Truncated N-terminus. Extended N-terminus.