Human casein kinase I1 (CKII) is a ubiquitous and multipotential Ser/Thr kinase involved in the regulation of cell growth and differentiation. Biochemically, two characteristics are particularly notable; first, the tetrameric composition of two catalytic subunits ( a and/or a') and two regulatory subunits @); second, the autophosphorylation of the holoenzyme at the N-terminus of CKIIP, suspected to be involved in tuning of the kinase activity. Whether CKIIa and CKIIa' reconstitute comparably with CKIIP to form holoenzyme is unclear. For a systematic investigation, the complete set of recombinant CKII subunits and of autophosphorylation mutants of C H I P were expressed in Escherichiu coli and comparative reconstitutions carried out. At 1 : 1 molar ratio, CKIIP stimulated both catalytic subunits roughly fivefold with phosvitin as a substrate. The level of activity reached with both of the reconstituted CKII isoforms was of the same order of magnitude as that of holoenzyme isolated from human placenta. It was also similar to a recombinant a,p2 holoenzyme whose expression had been attained in E. coli with a bicistronic construct containing the coding regions of CKIIP and CKIIa in a tandem arrangement. Both Ser2 and Ser3 were identified as the autophosphorylation sites ; replacement of one of these with Ala by oligonucleotide-mediated site-directed mutagenesis influenced only the extent of CKIIP autophosphorylation, replacement of both resulted in a loss of autophosphorylation. Despite these differences, the stimulatory effect of all the CKIIP mutants was comparable both to each other and to that of wild-type CKIIP. This was also obtained when substrates other than phosvitin were employed such as tubulin, or upstream-binding factor (UBF). However, the degree of stimulation was substrate specific and ranged from 2-5-fold with no major differences between CKIIa and CKIIa' stimulation. Calmodulin phosphorylation by both CKIIa and CKIIa' was decreased similarly by CKIIP and the CKIIP mutants. Proteins such as CAMP-1-esponsive-element-binding protein (CREB), HPVl6 E7 or Jun were not phosphorylated by either catalytic subunit but became substrates of both in the presence of CKIIP or CKIIP mutants. The data suggest that CKIIa and CKIIa' form similar CKII holoenzymes and that the tuning of holoenzyme activity is independent of the autophosphorylation status of CKIIP.Casein kinase type I1 (CKII) is one of three protein kinases named for their ability to phosphorylate acidic proteins such as casein. These are the authentic, mammary-gland-specific Golgi-located casein kinase and two operationally defined ubiquitous kinases, the casein kinase type I (CKI) and CKII. While Golgi-located CK definitely phosphorylates casein in vivo, CKI and CKII do not; these phosphorylate a vast array of cellular proteins including key factors of cell Correspondence to W. Pyerin, Biochemical Cell Physiology,

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In this study we determined the prevalence of autoantibodies against casein kinase II (CKII) in patients positive for anti-70K marker antibodies, which is indicative of mixed connective tissue disease. An anti-CKII ELISA was established with the use of bacterially expressed recombinant CKII proteins. Eight out of 52 anti-70K-positive sera (15%) were positive for anti-CKII antibodies, which recognized preferentially the CKII alpha subunit. All control sera (n = 52) were anti-CKII negative. Thus, the occurrence of anti-CKII antibodies may be of value for differential diagnosis.