Direct Regulatory Interactions Research Articles

Phospholamban Regulates the Ca2+-ATPase through Intramembrane Interactions

There is clear evidence for direct regulatory protein-protein interactions between phospholamban (PLN) and the Ca2+-ATPase of cardiac sarcoplasmic reticulum (SERCA2a) in cytoplasmic domains, but there is less clear evidence for regulatory interactions in the transmembrane domains of the two proteins. We have now coexpressed SERCA isoforms with the transmembrane sequence of PLN and with epitope-tagged transmembrane sequences of PLN to study intramembrane interactions in the absence of cytoplasmic interactions. Coexpression of the transmembrane sequence of phospholamban (Met-PLN28-52) with SERCA1a, SERCA2a, and SERCA3 inhibited Ca2+ transport by lowering apparent Ca2+ affinity. Addition of the hemagglutinin (HA) epitope to the transmembrane sequence of PLN (HA-PLN28-52) or deletion of PLN residues 21-29 (PLN1-20-PLN30-52) "supershifted" apparent Ca2+ affinity to values lower than those observed with native PLN without uncoupling Ca2+ transport from ATP hydrolysis. Inhibition by PLN1-20-PLN30-52 or by Flag-PLN28-52 was reversed by PLN antibody or by Flag antibody, demonstrating that inhibition by these constructs is reversible and that the inhibitory constructs are properly oriented in the membrane. These results suggest that PLN modulates the apparent Ca2+ affinity of SERCA2a through intramembrane interactions, which are disrupted at long range and in concert with disruption of the well characterized cytoplasmic interactions.

Transcription of ColE1Ap mbeC induced by conjugative plasmids from twelve different incompatibility groups.

Although nonconjugative mobilizable plasmids require helping functions of conjugative plasmids in order to be mobilized into recipients, at least some genes from the nonconjugative plasmids may be induced to assist in the DNA transfer process. Conjugative plasmids from 12 different incompatibility groups mobilized the nonconjugative plasmid ColE1Ap between Escherichia coli strains. Introduction of any of the conjugative plasmids into the ColE1Ap-containing strain resulted in an induction of mbeC, the product of which is a component of the mobilization relaxation complex. Each of the conjugative plasmids caused protein to bind specifically to mbe promoter DNA, suggesting a direct regulatory interaction.

Direct homeodomain-DNA interaction in the autoregulation of the fushi tarazu gene.

A major problem in the elucidation of the molecular mechanisms governing development is the distinction between direct and indirect regulatory interactions among developmental control genes. In vivo studies have indicated that the Drosophila segmentation gene fushi tarazu (ftz) directly or indirectly autoregulates its expression. Here we describe a generally applicable experimental approach which establishes a direct in vivo interaction of the homeodomain protein ftz with the ftz cis-autoregulatory control region. In vitro studies have shown that the DNA-binding specificity of the ftz homeodomain can be changed by a single amino-acid substitution in the recognition helix (Gln 50----Lys). Whereas wild-type ftz homeodomain binds preferentially to a CCATTA motif, the mutant homeodomain (ftzQ50K) recognizes a GGATTA motif. We now find that the in vivo activity of an ftz autoregulatory enhancer element is reduced by mutations of putative ftz-binding sites to GGATTA. This down-regulatory effect is specifically suppressed in vivo by the DNA-binding specificity mutant ftzQ50K. These results establish a direct positive autoregulatory feedback mechanism in the regulation of this homeobox gene.

Molecular interactions in the control region of the carAB operon encoding Escherichia coli carbamoylphosphate synthetase

The control region of the carAB operon, encoding carbamoylphosphate synthetase, comprises two tandem promoters (P1, upstream and P2, downstream) located 67 base-pairs apart and repressed respectively by pyrimidines and arginine. RNA polymerase and pure arginine repressor bind to the P2 region in mutually exclusive ways. Repressor protects the two adjacent palindromic ARG boxes overlapping P2 against DNase I. Binding of RNA polymerase to P1 is abnormal; the region protected against DNase I is shifted upstream by about 20 nucleotides with respect to the position expected from the transcription startpoint. This pattern is not due to interference with polymerase binding at P2, since it is observed also in the presence of repressor and on an isolated P1 region. Binding of RNA polymerase is relatively weak and heparin-sensitive suggesting that, in vivo, an ancillary factor is required to promote the formation of an open complex. S 1 nuclease mapping experiments show that the simultaneous presence of pyrimidines and arginine represses the downstream arginine-specific promoter (P2) more efficiently than arginine alone. This effect is not due to a direct regulatory interaction between pyrimidines and P2, since it is not observed when P1 is inactivated by insertion mutations or partial deletion. It has been shown that transcription initiated at P1 can proceed even when arginine represses P2. We therefore suggest that P2 operator-arginine repressor complex is destabilized by RNA polymerase binding at P1 or transcription from P1. We describe a novel technique to select for expression-down mutants in a lac fusion context.