Molecular cloning and characterization of a putative neural calcium channel alpha1-subunit from squid optic lobe

A Voltage-Gated Calcium-Selective Channel Encoded by a Sodium Channel-like Gene

BSC1, which was originally identified by its sequence similarity to voltage-gated Na(+) channels, encodes a functional voltage-gated cation channel whose properties differ significantly from Na(+) channels. BSC1 has slower kinetics of activation and inactivation than Na(+) channels, it is more selective for Ba(2+) than for Na(+), it is blocked by Cd(2+), and Na(+) currents through BSC1 are blocked by low concentrations of Ca(2+). All of these properties are more similar to voltage-gated Ca(2+) channels than to voltage-gated Na(+) channels. The selectivity for Ba(2+) is partially due to the presence of a glutamate in the pore-forming region of domain III, since replacing that residue with lysine (normally present in voltage-gated Na(+) channels) makes the channel more selective for Na(+). BSC1 appears to be the prototype of a novel family of invertebrate voltage-dependent cation channels with a close structural and evolutionary relationship to voltage-gated Na(+) and Ca(2+) channels.

Cloning and functional characterization of squid voltage-dependent Ca2+ channel beta subunits: involvement of N-terminal sequences in differential modulation of the current

cDNAs that encode beta subunits of voltage-dependent Ca(2+) channel were cloned from the optic lobe of the squid Loligo bleekeri. The subunits, LoCa(v)beta(1a) and LoCa(v)beta(1b) are 96% identical in amino acid sequence. The sole sequence differences are in the N-terminal region and in a five amino acid insertion in the central region of LoCa(v)beta(1b). RT-PCR revealed that LoCa(v)beta(1a) and LoCa(v)beta(1b) transcripts were expressed mainly in the optic lobe and stellate ganglion, and more weakly in mantle muscle, systemic heart, gill, branchial heart, stomach and liver. Coexpression of LoCa(v)beta(1a) or LoCa(v)beta(1b) with mammalian Ca(v)2.3 and alpha(2)/delta subunits in the Xenopus oocyte resulted in high-voltage-activated currents, and showed slow current inactivation and moderate steady-state inactivation. Comparison of the squid subunits with four mammalian beta subunits, beta(1b), beta(2a), beta(3) and beta(4), demonstrated that the modulatory effects of the beta subunits on steady-state inactivation kinetics were beta(3)<beta(4) approximately beta(1b)<LoCa(v)beta(1a) approximately LoCa(v)beta(1b)<beta(2a). LoCa(v)beta(1a)-induced current amplitude was about two to four times higher than that of LoCa(v)beta(1b). Experiments with point mutants and chimeras suggest that potential PKC and CK2 phosphorylation sites in the N-terminal region of LoCa(v)beta(1b) affect the current amplitude reciprocally, and may be responsible for regulating current amplitude.

Functional identification of a cloned squid presynaptic voltage-dependent calcium channel

We previously cloned a voltage-dependent Ca2+ channel alpha1 subunit LoCa(v)2 cDNA from the squid optic lobe. LoCa(v)2 is designated as a non-L-type voltage-dependent Ca2+ channel based on its amino acid sequence. We performed functional expression experiments of LoCa(v)2 in oocytes and characterized the expressed currents electrophysiologically and pharmacologically. The LoCa(v)2 current was high voltage-activated and the peak current was maximal at +20 mV and lasted for long during activation. The LoCa(v)2 current was not inhibited by the drugs and toxins examined except for omega-agatoxin IVA and PLTX-II. Omega-agatoxin IVA, which is a P-type channel blocker, moderately inhibited the LoCa(v)2 current at higher concentration. PLTX-II, which blocks insect presynaptic Ca2+ channel, inhibited the LoCa(v)2 current at lower concentration. Immunohistochemical investigation showed that the LoCa(v)2 protein may exist at presynaptic terminals in the squid optic lobe. These results suggest that LoCa(v)2 is an omega-agatoxin IVA and PLTX-II-sensitive presynaptic Ca2+ channel in the squid nervous system.

The molecular biology of invertebrate voltage-gated Ca(2+) channels

The importance of voltage-gated Ca(2+) channels in cellular function is illustrated by the many distinct types of Ca(2+) currents found in vertebrate tissues, a variety that is generated in part by numerous genes encoding Ca(2+) channel subunits. The degree to which this genetic diversity is shared by invertebrates has only recently become apparent. Cloning of Ca(2+) channel subunits from various invertebrate species, combined with the wealth of information from the Caenorhabditis elegans genome, has clarified the organization and evolution of metazoan Ca(2+) channel genes. Functional studies have employed novel structural information gained from invertebrate Ca(2+) channels to complement ongoing research on mammalian Ca(2+) currents, while demonstrating that the strict correspondence between pharmacological and molecular classes of vertebrate Ca(2+) channels does not fully extend to invertebrate tissues. Molecular structures can now be combined with physiological data to develop a more cogent system of categorizing invertebrate channel subtypes. In this review, we examine recent progress in the characterization of invertebrate Ca(2+) channel genes and its relevance to the diversity of invertebrate Ca(2+) currents.

Association of neuronal calcium channels with modular adaptor proteins

Presynaptic voltage-gated calcium (Ca(2+)) channels mediate Ca(2+) influx into the presynaptic terminal that triggers synaptic vesicle fusion and neurotransmitter release. The immediate proximity of Ca(2+) channels to the synaptic vesicle release apparatus is critical for rapid and efficient synaptic transmission. In a series of biochemical experiments, we demonstrate a specific association of the cytosolic carboxyl terminus of the N-type Ca(2+) channel pore-forming alpha(1B) subunit with the modular adaptor proteins Mint1 and CASK. The carboxyl termini of alpha(1B) bind to the first PDZ domain of Mint1 (Mint1-1). The proline-rich region present in the carboxyl termini of alpha(1B) binds to the SH3 domain of CASK. Mint1-1 is specific for the E/D-X-W-C/S-COOH consensus, which defines a novel class of PDZ domains (class III). The Mint1-1 PDZ domain-binding motif is present only in the "long" carboxyl-terminal splice variants of N-type (alpha(1B)) and P/Q-type (alpha(1A)) Ca(2+) channels, but not in R-type (alpha(1E)) or L-type (alpha(1C)) Ca(2+) channels. Our results directly link presynaptic Ca(2+) channels to a macromolecular complex formed by modular adaptor proteins at synaptic junction and advance our understanding of coupling between cell adhesion and synaptic vesicle exocytosis.

Cloning and characterization of a cDNA encoding a novel fibroblast growth factor preferentially expressed in human heart

A novel human fibroblast growth factor (hFGF), which shows 75% sequence homology with fibroblast growth factor-9, was isolated in random sequencing of a human heart cDNA library. The full-length sequence is 928 bp, the encoded protein is composed of 168 amino acid residues, and its pI value and molecular weight were estimated to be 8.13 and 19.1 kDa, respectively. RT-PCR using Marathon human heart cDNA shows that the coding region is approximately 507 bp. Southern hybridization showed a single band which indicates that this is a single copy gene. Northern hybridization done on a human multiple tissues blot showed that the gene is preferentially expressed in human heart, very weakly detectable in human brain and not detectable in 18 other different human tissues.

Cloning and functional expression of a voltage-gated calcium channel alpha1 subunit from jellyfish

Voltage-gated Ca2+ channels in vertebrates comprise at least seven molecular subtypes, each of which produces a current with distinct kinetics and pharmacology. Although several invertebrate Ca2+ channel alpha1 subunits have also been cloned, their functional characteristics remain unclear, as heterologous expression of a full-length invertebrate channel has not previously been reported. We have cloned a cDNA encoding the alpha1 subunit of a voltage-gated Ca2+ channel from the scyphozoan jellyfish Cyanea capillata, one of the earliest existing organisms to possess neural and muscle tissue. The deduced amino acid sequence of this subunit, named CyCaalpha1, is more similar to vertebrate L-type channels (alpha1S, alpha1C, and alpha1D) than to non-L-type channels (alpha1A, alpha1B, and alpha1E) or low voltage-activated channels (alpha1G). Expression of CyCaalpha1 in Xenopus oocytes produces a high voltage-activated Ca2+ current that, unlike vertebrate L-type currents, is only weakly sensitive to 1,4-dihydropyridine or phenylalkylamine Ca2+ channel blockers and is not potentiated by the agonist S(-)-BayK 8644. In addition, the channel is less permeable to Ba2+ than to Ca2+ and is more permeable to Sr2+. CyCaalpha1 thus represents an ancestral L-type alpha1 subunit with significant functional differences from mammalian L-type channels.