Biochemical and cellular characteristics of the four splice variants of protein kinase CK1 α from zebrafish (Danio rerio)

Protein kinase CK1 (previously known as casein kinase I) conforms to a subgroup of the great protein kinase family found in eukaryotic organisms. The CK1 subgroup of vertebrates contains seven members known as α, β, γ1, γ2, γ3, δ and ε. The CK1 α gene can generate four variants (CK1 α, CK1 αS, CK1 αL, and CK1 αLS) through alternate splicing, characterized by the presence or absence of two additional coding sequences. Exon "L" encodes a 28-amino acid stretch that is inserted after lysine 152, in the center of the catalytic domain. The "S" insert encodes 12 amino acid residues and is located close to the carboxyl terminus of the protein. This work reports some biochemical and cellular properties of the four CK1 α variants found to be expressed in zebrafish (Danio rerio). The results obtained indicate that the presence of the "L" insert affects several biochemical properties of CK1 α: (a) it increases the apparent Km for ATP twofold, from, ∼30 to ∼60 μM; (b) it decreases the sensitivity to the CK1-7 inhibitor, raising the I₅₀ values from 113 to ∼230 μM; (c) it greatly decreases the heat stability of the enzyme at 40°C. In addition, the insertion of the "L" fragment exerts very important effects on some cellular properties of the enzyme. CK1 αL concentrates in the cell nucleus, excluding nucleoli, while the CK1 α variant is predominantly cytoplasmic, although some presence is observed in the nucleus. This finding supports the thesis that the basic-rich region found in the "L" insert acts as a nuclear localization signal. The "L" insert-containing variant was also found to be more rapidly degraded (half-life of 100 min) than the CK1 α variant (half-life of 400 min) in transfected Cos-7 cells.