MinK is a widely expressed protein of relative molecular mass approximately 15K that forms potassium channels by aggregation with other membrane proteins. MinK governs ion channel activation, regulation by second messengers, and the function and structure of the ion conduction pathway. Association of minK with a channel protein known as KvLQT1 produces a voltage-gated outward K+ current (I[sK]) resembling the slow cardiac repolarization current (I[Ks]). HERG, a human homologue of the ether-a-go-go gene of the fruitfly Drosophila melanogaster, encodes a protein that produces the rapidly activating cardiac delayed rectifier (I[Kr]). These two potassium currents, I(Ks) and I(Kr), provide the principal repolarizing currents in cardiac myocytes for the termination of action potentials. Although heterologously expressed HERG channels are largely indistinguishable from native cardiac I(Kr), a role for minK in this current is suggested by the diminished I(Kr) in an atrial tumour line subjected to minK antisense suppression. Here we show that HERG and minK form a stable complex, and that this heteromultimerization regulates I(Kr) activity. MinK, through the formation of heteromeric channel complexes, is thus central to the control of the heart rate and rhythm.

Sorcin is a 22-kDa calcium-binding protein initially identified in multidrug-resistant cells; however, its patterns of expression and function in normal tissues are unknown. Here we demonstrate that sorcin is widely distributed in rodent tissues, including the heart, where it was localized by immunoelectron microscopy to the sarcoplasmic reticulum. A >500-kDa protein band immunoprecipitated from cardiac myocytes by sorcin antiserum was indistinguishable in size on gels from the 565-kDa ryanodine receptor/calcium release channel recognized by ryanodine receptor-specific antibody. Association of sorcin with a ryanodine receptor complex was confirmed by complementary co-immunoprecipitations of sorcin with the receptor antibody. Forced expression of sorcin in ryanodine receptor-negative Chinese hamster lung fibroblasts resulted in accumulation of the predicted 22-kDa protein as well as the unexpected appearance of ryanodine receptor protein. In contrast to the parental host fibroblasts, sorcin transfectants displayed a rapid and transient rise in intracellular calcium in response to caffeine, suggesting organization of the accumulated ryanodine receptor protein into functional calcium release channels. These data demonstrate an interaction between sorcin and the ryanodine receptor and suggest a role for sorcin in modulation of calcium release channel activity, perhaps by stabilizing the channel protein.Sorcin was initially identified as a 22-kDa protein in cultured cells selected for resistance to natural product cancer drugs, such as vincristine, adriamycin, and actinomycin D, i.e. multidrug-resistant cells (1-5). One of the major mechanisms of resistance in these cells is mediated by overexpression of the membrane-bound drug transporter, P-glycoprotein (6). Molecular cloning studies demonstrated that the sorcin and P-glycoprotein genes are tightly linked and that both may be amplified during the acquisition of multidrug resistance. However, while P-glycoprotein overexpression correlates with resistance development, increased sorcin expression is not obligatory, and its abundance does not correlate with the degree of resistance (4 -9). Complementary DNA for sorcin has been isolated from hamster (2) and human (10) multidrug-resistant cells, which amplify the sorcin gene. The highly conserved sequence, with 95% homology between hamster and human sorcin, predicts a 22-kDa protein with four putative Ca 2ϩ

is not an obligatory factor for sorcin inhibition of RyR. Comparisons of these inhibitory effects with those of calmodulin and calpain, proteins structurally related to sorcin, suggested that the interaction of sorcin with cardiac RyR was distinct from and independent of either of these modulatory proteins. Phosphorylation of sorcin with the catalytic subunit of protein kinase A significantly decreased the ability of sorcin to modulate RyR. These results suggest that sorcin may modulate RyR function in a normal cell environment and that the level of modulation is in turn influenced by signaling pathways that increase protein kinase A activity.

We have previously shown that at least five linked genes are co‐amplified and overexpressed in the multi‐drug resistant (MDR) Chinese hamster ovary cell line CHRC5. We show here that one of these genes (class 4) codes for a small phosphorylated, cytosolic protein, sorcin/V19, known to be overproduced by many MDR cell lines. The class 4 gene codes for a nested set of mRNAs, varying in size between 1000 and 2500 nucleotides. Sequence analysis of complementary DNAs shows that these mRNAs encode a protein of 198 amino acids. The identity of this protein with sorcin was established by comparison with the amino acid sequence of two peptides from mouse sorcin. Hamster sorcin is a 22‐kd protein with four ‘E‐F hand’ structures typical of calcium‐binding sites and it has substantial homology with the light chain of calpain. Two of the calcium‐binding sites contain putative recognition sites for cAMP‐dependent protein kinase. These may account for the known phosphorylation of sorcin. The unknown function of sorcin might therefore be controlled by both calcium and cAMP levels. The contribution of sorcin to multidrug resistance, if any, remains to be tested.

Sorcin, a 22 kDa calcium binding protein present in abundance in cardiac tissue and in multi-drug resistant cells and previously described as a soluble protein, is now shown to undergo a calcium-dependent translocation process from the cytosol to cellular membranes in both systems. The translocation process takes place also in E. coli BL21 cells that express recombinant sorcin, r-sorcin, and can be exploited in the purification of the protein. Calcium binding to purified r-sorcin occurs at micromolar concentrations of the metal and is accompanied by a conformational change that renders the protein soluble in the non-ionic detergent Triton X-114. This finding suggests that lipids are the target of sorcin on cellular membranes. The possible significance of the calcium-dependent translocation of sorcin in the specialized functions of sorcin-expressing cells is discussed.