Comparative analysis of inactivated-state block of N-type (Ca(v)2.2) calcium channels.

OBJECTIVE: The aim of this study was to compare a diverse set of peptide and small-molecule calcium channel blockers for inactivated-state block of native and recombinant N-type calcium channels using fluorescence-based and automated patch-clamp electrophysiology assays. METHODS: The pharmacology of calcium channel blockers was determined at N-type channels in IMR-32 cells and in HEK cells overexpressing the inward rectifying K(+) channel Kir2.1. N-type channels were opened by increasing extracellular KCl. In the Kir2.1/N-type cell line the membrane potential could be modulated by adjusting the extracellular KCl, allowing determination of resting and inactivated-state block of N-type calcium channels. The potency and degree of state-dependent inhibition of these blockers were also determined by automated patch-clamp electrophysiology. RESULTS: N-type-mediated calcium influx in IMR-32 cells was determined for a panel of blockers with IC(50) values of 0.001-7 µM and this positively correlated with inactivated-state block of recombinant channels measured using electrophysiology. The potency of several compounds was markedly weaker in the state-dependent fluorescence-based assay compared to the electrophysiology assay, although the degree of state-dependent blockade was comparable. CONCLUSIONS: The present data demonstrate that fluorescence-based assays are suitable for assessing the ability of blockers to selectively interact with the inactivated state of the N-type channel.

Structural analysis of UBL5, a novel ubiquitin-like modifier.

UBL5 is a widely expressed human protein that is strongly conserved across phylogeny. Orthologs of UBL5 occur in every eukaryotic genome characterized to date. The yeast ortholog of UBL5, HUB1, was reported to be a ubiquitin-like protein modifier important for modulation of protein function. However, unlike ubiquitin and all other ubiquitin-like modifiers, UBL5 and its yeast ortholog HUB1 both contain a C-terminal di-tyrosine motif followed by a single variable residue instead of the characteristic di-glycine found in all other ubiquitin-like modifiers. Here we describe the three-dimensional structure of UBL5 determined by NMR. The overall structure of the protein was found to be very similar to ubiquitin despite the low approximately 25% residue similarity. The signature C-terminal di-tyrosine residues in UBL5 are involved in the final beta sheet of the protein. This is very different to the di-glycine motif found in ubiquitin, which extends beyond the final beta sheet. In addition, we have confirmed an earlier report of an interaction between UBL5 and the cyclin-like kinase, CLK4, which we have determined is specific and does not extend to other cyclin-like kinase family members.