ECTO- AND EXO-PROTEIN KINASES IN SCHISTOSOMA MANSONI: REGULATION OF SURFACE PHOSPHORYLATION BY ACETYLCHOLINE AND IDENTIFICATION OF THE ALPHA SUBUNIT OF CKII AS A MAJOR SECRETED PROTEIN KINASE

Malaria Parasite-Infected Erythrocytes Secrete PfCK1, the Plasmodium Homologue of the Pleiotropic Protein Kinase Casein Kinase 1

Casein kinase 1 (CK1) is a pleiotropic protein kinase implicated in several fundamental processes of eukaryotic cell biology. Plasmodium falciparum encodes a single CK1 isoform, PfCK1, that is expressed at all stages of the parasite’s life cycle. We have previously shown that the pfck1 gene cannot be disrupted, but that the locus can be modified if no loss-of-function is incurred, suggesting an important role for this kinase in intra-erythrocytic asexual proliferation. Here, we report on the use of parasite lines expressing GFP- or His-tagged PfCK1 from the endogenous locus to investigate (i) the dynamics of PfCK1 localisation during the asexual cycle in red blood cells, and (ii) potential interactors of PfCK1, so as to gain insight into the involvement of the enzyme in specific cellular processes. Immunofluorescence analysis reveals a dynamic localisation of PfCK1, with evidence for a pool of the enzyme being directed to the membrane of the host erythrocyte in the early stages of infection, followed by a predominantly intra-parasite localisation in trophozoites and schizonts and association with micronemes in merozoites. Furthermore, we present strong evidence that a pool of enzymatically active PfCK1 is secreted into the culture supernatant, demonstrating that PfCK1 is an ectokinase. Our interactome experiments and ensuing kinase assays using recombinant PfCK1 to phosphorylate putative interactors in vitro suggest an involvement of PfCK1 in many cellular processes such as mRNA splicing, protein trafficking, ribosomal, and host cell invasion.

How druggable is protein kinase CK2?

CK2 is a pleiotropic, ubiquitous, and constitutively active protein kinase (PK), with both cytosolic and nuclear localization in most mammalian cells. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. CK2 catalyzes the phosphorylation of more than 300 substrates characterized by multiple acidic residues surrounding the phosphor-acceptor amino acid, and, consequently, it plays a key role in several physiological and pathological processes. But how can one kinase orchestrate all these tasks faithfully? How is it possible that one kinase can, despite all pleiotropic characteristics of PKs in general, be involved in so many different biochemical events? Is CK2 a druggable target? Several questions are still to be clearly answered, and this review is an occasion for a fruitful discussion.

Cell surface phosphorylation by a novel ecto-protein kinase: a key regulator of cellular functions in spermatozoa

Since 1976 many studies have been reported on the occurrence and functional significance of ecto-protein kinases in a variety of cell types although their precise biochemical identity is largely unknown. This study reports for the first time purification to apparent homogeneity of an ecto-protein kinase (ecto-CIK) and some of its characteristics using caprine sperm as the cell model. The ecto-CIK is a unique membrane-specific serine/threonine protein kinase. It is a strongly basic 115 kDa protein made up of two subunits: 63 and 55 kDa. The ecto-kinase undergoes a remarkable lateral movement on the outer cell surface culminating in capping on the sperm acrosomal tip. MPS, its major protein substrate is also located on the acrosomal tip. Both ecto CIK and MPS serve as potential regulators of flagellar motility. This novel enzyme appears to be major kinase responsible for the reported regulation of mammalian cellular functions by modulating phosphorylation of the membrane-bound proteins.