Effects of CapG overexpression on agonist-induced motility and second messenger generation.

Actin modulating proteins that bind polyphosphoinositides, such as phosphatidylinositol 4, 5-bisphosphate (PIP2), can potentially participate in receptor signaling by restructuring the membrane cytoskeleton and modulating second messenger generation through the phosphoinositide cycle. We examined these possibilities by overexpressing CapG, an actin filament end capping, Ca(2+)- and polyphosphoinositide-binding protein of the gelsolin family. High level transient overexpression decreased actin filament staining in the center of the cells but not in the cell periphery. Moderate overexpression in clonally selected cell lines did not have a detectible effect on actin filament content or organization. Nevertheless, it promoted a dose-dependent increase in rates of wound healing and chemotaxis. The motile phenotype was similar to that observed with gelsolin overexpression, which in addition to capping, also severs and nucleates actin filaments. CapG overexpressing clones are more responsive to platelet-derived growth factor than control-transfected clones. They form more circular dorsal membrane ruffles, have higher phosphoinositide turnover, inositol 1,4,5-trisphosphate generation and Ca2+ signaling. These responses are consistent with enhanced PLC gamma activity. Direct measurements of PIP2 mass showed that the CapG effect on PLC gamma was not due primarily to an increase in the PIP2 substrate concentration. The observed changes in cell motility and membrane signaling are consistent with the hypothesis that PIP(2)-binding actin regulatory proteins modulate phosphoinositide turnover and second messenger generation in vivo. We infer that CapG and related proteins are poised to coordinate membrane signaling with actin filament dynamics following cell stimulation.

The actin side-binding domain of gelsolin also caps actin filaments. Implications for actin filament severing.

Gelsolin is an actin filament-severing and -capping protein which is inhibited by polyphosphoinositides (PPI). Severing requires gelsolin binding to the side of the filaments through a site in segments 2 and 3 (S2-3) to position another site in segment 1 (S1) to sever filaments. In this paper, we report that S2-3, like S1, caps actin filaments. Since neither S1 and S2-3 caps as well as gelsolin, and neither severs actin filament, S2-3 may actively contribute to severing by capping filaments cooperatively with S1. We used deletional mutagenesis to locate the S2-3 sequence required for actin filament side binding, capping, and PPI binding and found that these sites are located close to the NH2 terminus of S2 (residues 161-172). S3, a segment which has no known function up to now and does not by itself bind actin, contributes to stable capping and may contain an additional PPI-binding site.

Calcium gating of H+ fluxes in chloroplasts affects acid-base-driven ATP formation.

In previous work, calcium ions, bound at the lumenal side of the CF0H+ channel, were suggested to keep a H+ flux gating site closed, favoring sequestered domain H+ ions flowing directly into the CF0-CF1 and driving ATP formation by a localized delta approximately mu H+ gradient. Treatments expected to displace Ca++ from binding sites had the effect of allowing H+ ions in the sequestered domains to equilibrate with the lumen, and energy coupling showed delocalized characteristics. The existence of such a gating function implies that a closed-gate configuration would block lumenal H+ ions from entering the CF0-CF1 complex. In this work that prediction was tested using as an assay the dark, acid-base jump ATP formation phenomenon driven by H+ ions derived from succinic acid loaded into the lumen. Chlorpromazine, a photoaffinity probe for many proteins having high-affinity Ca(++)-binding sites, covalently binds to the 8-kDa CF0 subunit in the largest amounts when there is sufficient Ca++ to favor the localized energy coupling mode, i.e., the "gate closed" configuration. Photoaffinity-bound chlorpromazine blocked 50% or more of the succinate-dependent acid-base jump ATP formation, provided that the ionic conditions during the UV photoaffinity treatment were those which favor a localized energy coupling pattern and a higher level of chlorpromazine labeling of the 8-kDa CF0 subunit. Thylakoids held under conditions favoring a delocalized energy coupling mode and less chlorpromazine labeling of the CF0 subunit did not show any inhibition of acid-base jump ATP formation. Chlorpromazine and calmidazolium, another Ca(++)-binding site probe, were also shown to block redox-derived H+ initially released into sequestered domains from entering the lumen, at low levels of domain H+ accumulation, but not at higher H+ uptake levels; ie., the closed gate state can be overcome by sufficiently acidic conditions. That is consistent with the observation that the inhibition of lumenal succinate-dependent ATP formation by photoaffinity-attached chlorpromazine can be reversed by lowering the pH of the acid stage from 5.5 to 4.5. The evidence is consistent with the concept that Ca++ bound at the lumenal side of the CF0 H+ channel can block H+ flux from either direction, consistent with the existence of a molecular structure in the CF0 complex having the properties of a gate for H+ flux across the inner boundary of the CF0. Such a gate could control the expression of localized or delocalized delta approximately mu H+ energy coupling gradients.

Calcium-dependent interaction of chlorpromazine with the chloroplast 8-kilodalton CF0 protein and calcium gating of H+ fluxes between thylakoid membrane domains and the lumen.

Earlier work suggested that Ca2+ ions in the chloroplast thylakoid lumen interact with thylakoid membrane proteins to produce a proton flux gating structure which functions to regulate the expression of the energy-coupling H+ gradient between localized and delocalized modes [Chiang, G., & Dilley, R. A. (1987) Biochemistry 26, 4911-4916]. In this work, one of the phenothiazine Ca2+ antagonists, chlorpromazine, was used as a photoaffinity probe to test for Ca(2+)-dependent binding of the probe to thylakoid proteins. [3H]Chlorpromazine photoaffinity-labels thylakoid polypeptides of Mr 8K and 6K, with generally much less label occurring in other proteins (some experiments showed labeled proteins at Mr 13K-15K). More label was incorporated in circumstances where it is expected that Ca2+ occupies the putative H+ flux gating site, compared to when the gating site is not occupied by calcium. The photoaffinity labeling of the 8-kDa protein was also influenced by the energization level of the thylakoids (less labeling under H+ uptake energization). The 8-kDa protein was identified by partial amino acid sequence data as subunit III of the thylakoid CF0 H+ channel complex. The partial amino acid sequence of the 6-kDa protein (19 residues were determined with some uncertainties) was compared to data in the GCG sequence analysis data base, and no clear identity to a known sequence was revealed. Neither the exact site of putative Ca2+ binding to the CF0 proteolipid nor the site of covalent attachment of the chlorpromazine to the CF0 component has been identified. Evidence for gating of energy-linked H+ fluxes by the hypothesized Ca(2+)-CF0 gating site came from the correlation between Ca(2+)-dependent binding of chlorpromazine to the CF0 8-kDa protein with inhibition of light-driven H+ uptake into the lumen but no inhibition of H+ uptake into sequestered membrane domains. When conditions favored a delocalized delta mu H+ coupling mode, less chlorpromazine was bound to the CF0 structure, and much larger amounts of H+ ions were accumulated in the lumen. The data support the hypothesis that Ca2+ ions act in concert with the 8-kDa CF0 protein (and perhaps another protein, the 6-kDa polypeptide?) in a gating mechanism for regulating the expression of the energy-coupling H+ gradient between localized or delocalized coupling modes.