Molecular cloning of human macrophage capping protein cDNA. A unique member of the gelsolin/villin family expressed primarily in macrophages

Analysis of a sequenced cDNA library from multiple sclerosis lesions

To identify genes that are expressed in MS pathogenesis, we have analyzed a normalized cDNA library made from mRNA obtained from CNS lesions of a patient with primary progressive MS. Complementary DNA clones obtained from this library were subjected to automated DNA sequencing to generate expressed sequence tags. Analysis of this MS cDNA library revealed the presence of 54 cDNAs that were associated with immune activation and indicated the presence of an ongoing inflammatory response with evidence of both cell-mediated and humoral immune responses. The surprising finding was that 16 of the cDNAs encoded autoantigens associated with seven other autoimmune disorders, while only three of these 16 autoantigen cDNAs were present in a similarly constructed adult brain library. Such aberrant autoantigen expression could provide a source of secondary autoimmune stimulation that could contribute to the ongoing inflammatory response in MS. In addition, two cDNAs were found that mapped to a known MS susceptibility locus (5p14-p12): one encoded an excitatory amino acid transporter and the other a human homologue of the Drosophila disabled gene. This approach to the molecular biology of MS pathogenesis may help to illuminate previously unappreciated aspects of this disease.

Purification and properties of a Ca(2+)-independent barbed-end actin filament capping protein, CapZ, from human polymorphonuclear leukocytes

In human polymorphonuclear leukocytes (PMN), changes in the actin architecture are critical for the shape changes required for chemotaxis and phagocytosis. Barbed-end capping proteins are likely to regulate actin assembly in PMN. The previously identified barbed-end blocking proteins in PMN, gelsolin and CapG, require Ca(2+) to initiate capping of actin filaments. Because chemoattractants can stimulate PMN actin assembly by a calcium-independent signal transduction pathway, we sought to purify a calcium-independent barbed-end capping activity from PMN cytoplasmic extracts. A Ca(2+) -insensitive actin polymerization inhibitory activity was partially purified from human PMN [Southwick & Stossel (1981) J. Biol. Chem 256, 3030]. Using five column chromatography steps, we purified the protein to homogeneity as assessed by silver staining. Purification was associated with an increase in specific activity of greater than 40 X. Western blot analysis identified the protein as the nonmuscle isoform of the heterodimeric capping protein capZ. Human PMN capZ has an apparent disassociation constant of 3 nM for capping in the presence or absence of micromolar Ca(2+), as assessed by both pyrenylactin elongation and depolymerization assays. Similar to the activity reported for the actin polymerization inhibitor, activity of PMN capZ was inhibited by increasing the KC1 concentration from 0.1 M to 0.6 M. The capping function was also inhibited by phosphatidylinositol 4,5-bisphosphate (PIP(2)) micelles, with half-maximal inhibition occurring at 5.5 micrograms mL(-1). PMN capZ did not nucleate actin assembly, sequester actin monomers, or sever actin filaments. Quantitative Western blot analysis revealed that capZ levels corresponded to 0.7-1.0% of the total human PMN cytoplasmic protein. Given its abundance and high affinity for barbed filament ends, capZ is likely to play an important role in the calcium-independent regulation of actin filament assembly associated with PMN chemotaxis.

Sequence analysis and chromosomal localization of human Cap Z. Conserved residues within the actin-binding domain may link Cap Z to gelsolin/severin and profilin protein families

From a human retinal cDNA library, we have isolated cDNAs that are homologs for the alpha 2 and beta subunits of chicken Cap Z. The derived human alpha subunit shares 95% amino acid identity with the chicken alpha 2 subunit; the beta subunit is 99% identical to the chicken subunit residues 1-243. The remaining portion of the human beta subunit (244-272) diverges significantly with only 8 out of 29 C-terminal amino acids conserved between the two species. This lack of conservation is of particular interest because the chicken C terminus contains an actin-binding domain. Cosedimentation assays with F-actin show that human Cap Z binds actin with an affinity equal that of chicken Cap Z. These results point to the eight shared amino acids as critical for actin binding, three of which are regularly spaced leucines. These apolar residues and one outside the region of divergence align well with those residues of the actin-binding alpha-helix proposed for gelsolin segment 1. The apolar residues as well as three polar amino acids are also conserved in other capping, capping and severing, and monomer-binding proteins. Amino acid substitutions in the chicken beta subunit of the two most highly conserved leucines result in significant decreases in F-actin binding activity. The human alpha 2 gene (CAPZA2) has been mapped to chromosome 7 position q31.2-q31.3 and the beta gene (CAPZB) to chromosome 1 region p36.1.

Identification of the trapped calcium in the gelsolin segment 1-actin complex: implications for the role of calcium in the control of gelsolin activity

The X-ray structure of the complex of actin with gelsolin segment 1 revealed the presence of two calcium ions, one bound at an intramolecular site within segment 1 and the other bridging the segment directly to actin. Although earlier calcium binding studies at pH 8.0 revealed only a single calcium trapped in the complex (and also in the binary gelsolin-actin complex), it is here shown that two calcium ions are bound under the conditions of crystallization at physiological pH. Mutation of acidic residues in either actin or segment 1 involved in ligation of the intermolecular calcium ion resulted in loss of one of the bound calcium ions at pH < 7, but not at pH 8. Thus the calcium ion trapped in the segment 1-actin complex is that located at the intramolecular site. The implications of this for gelsolin function are discussed.

Gain-of-function mutations conferring actin-severing activity to human macrophage cap G

Nonmuscle cell motility requires marked changes in the consistency and shape of the peripheral cytoplasm. These changes are regulated by a gel-sol transformation of the actin filament network, and actin filament-severing proteins are responsible for network solation. Macrophage Cap G, unlike all other proteins in the gelsolin family, caps but does not sever actin filaments. Two amino acid stretches in Cap G diverge markedly from the severing proteins: 84LNTLLGE and 124AFHKTS. Discrete mutations in Cap G have been generated to determine if these amino acid sequences are critical for actin filament severing. Conversion of 84LNTLLGE to the gelsolin actin-binding helix sequence (84LDDYLGG) renders Cap G capable of severing actin filaments (half-maximal severing, 1-2 microM). Adding a second set of mutations, converting 124AFHKTS to 124GFKHV, enhances severing by 10-fold (half-maximal severing, 0.1-0.2 microM). These experiments support a critical role for these two regions in actin filament severing and showcase the power of gain-of-function mutations in clarifying structure-function relationships.

Dynamic remodeling of the actin cytoskeleton: lessons learned from Listeria locomotion

The bacterial pathogen Listeria monocytogenes displays the remarkable ability to reorganize the actin cytoskeleton within host cells as a means for promoting cell-to-cell transfer of the pathogen, in a manner that evades humoral immunity. In a series of events commencing with the biosynthesis of the bacterial surface protein ActA, host cell actin and many actin-associated proteins self-assemble to form rocket-tail structures that continually grow at sites proximal to the bacterium and depolymerize distally. Widespread interest in the underlying molecular mechanism of Listeria locomotion stems from the likelihood that the dynamic remodeling of the host cell actin cytoskeleton at the cell's leading edge involves mechanistically analogous interactions. Recent advances in our understanding of these fundamental cytoskeletal rearrangements have been achieved through a clearer recognition of the central role of oligo-proline sequence repeats present in ActA, and these findings provide a basis for inferring the role of analogous host cell proteins in the force-producing and position-securing steps in pseudopod and lamellipod formation at the peripheral membrane.

A qualitative and quantitative protein database approach identifies individual and groups of functionally related proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes: an overview of the functional changes associated with the transformed phenotype

A qualitative and quantitative two-dimensional (2-D) gel database approach has been used to identify individual and groups of proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes (K14). Five hundred and sixty [35S]methionine-labeled proteins (462 isoelectric focusing, IEF; 98 nonequilibrium pH gradient electrophoresis, NEPHGE), out of the 3038 recorded in the master keratinocyte database, were excised from dry, silver-stained gels of normal proliferating primary keratinocytes and K14 cells and the radioactivity was determined by liquid scintillation counting. Two hundred and thirty five proteins were found to be either up- (177) or down-regulated (58) in the transformed cells by 50% or more, and of these, 115 corresponded to known proteins in the keratinocyte database (J.E. Celis et al., Electrophoresis 1993, 14, 1091-1198). The lowest abundance acidic protein quantitated was present in about 60,000 molecules per cell, assuming a value of 10(8) molecules per cell for total actin. The results identified individual, and groups of functionally related proteins that are differentially regulated in K14 keratinocytes and that play a role in a variety of cellular activities that include general metabolism, the cytoskeleton, DNA replication and cell proliferation, transcription and translation, protein folding, assembly, repair and turnover, membrane traffic, signal transduction, and differentiation. In addition, the results revealed several transformation sensitive proteins of unknown identity in the database as well as known proteins of yet undefined functions. Within the latter group, members of the S100 protein family--whose genes are clustered on human chromosome 1q21--were among the highest down-regulated proteins in K14 keratinocytes. Visual inspection of films exposed for different periods of time revealed only one new protein in the transformed K14 keratinocytes and this corresponded to keratin 18, a cytokeratin expressed mainly by simple epithelia. Besides providing with the first global overview of the functional changes associated with the transformed phenotype of human keratinocytes, the data strengthened previous evidence indicating that transformation results in the abnormal expression of normal genes rather than in the expression of new ones.

Regulation of leukocyte locomotion by Ca2+

Neutrophils migrate towards sites of inflammation and infection by chemotaxis. Their motility is dependent on the actin cytoskeleton and on adhesion to extracellular substrates, but how these are regulated in response to stimuli is not clear. This review focuses on the potential role of Ca(2+) as a second messenger in neutrophil motility. Several effects of Ca(2+) and Ca(2+)-binding proteins on the stability and crosslinking of actin polymers have been demonstrated in vitro. Nevertheless, the complex mechanism by which Ca(2+) regulates actin in neutrophils is not fully understood. In addition, intracellular Ca(2+) regulates the intergin-mediated adhesion of neutrophils to extracellular matrix.

On the crawling of animal cells

Cells crawl in response to external stimuli by extending and remodeling peripheral elastic lamellae in the direction of locomotion. The remodeling requires vectorial assembly of actin subunits into linear polymers at the lamella's leading edge and the crosslinking of the filaments by bifunctional gelation proteins. The disassembly of the crosslinked filaments into short fragments or monomeric subunits away from the leading edge supplies components for the actin assembly reactions that drive protrusion. Cellular proteins that respond to lipid and ionic signals elicited by sensory cues escort actin through this cycle in which filaments are assembled, crosslinked, and disassembled. One class of myosin molecules may contribute to crawling by guiding sensory receptors to the cell surface, and another class may contribute by imposing contractile forces on actin networks in the lamellae.

F-actin capping proteins

Recent research on F-actin capping proteins has concentrated on three main areas. The discovery that controlled actin polymerization is the driving force for intracellular movement suggests an important role for capping proteins in regulating filament number and length. A capping protein from Dictyostelium (related to heat-shock protein HSP70) has been characterized that is activated by external stimuli. This provides a pivotal connection between extracellular signalling, cytoskeletal reorganization and locomotory behaviour. The roles of individual actin-binding sites in the gelsolin family of severing/capping proteins and binding sites for phosphatidylinositol 4,5-bisphosphate have been identified.

gCap39 is a nuclear and cytoplasmic protein

gCap39 is a newly identified member of the Ca(2+)- and polyphosphoinositide-modulated gelsolin family of actin binding proteins which is different from gelsolin in several important respects: it caps filament ends, it does not sever filaments, it binds reversibly to actin, it is phosphorylated in vivo, and it is also present in the nucleus. gCap39 and gelsolin coexist in a variety of cells. To better understand the roles of gCap39 and gelsolin, we have compared their relative amounts and intracellular distributions. We found that gCap39 is very abundant in macrophages (accounting for 0.6% of total macrophage proteins), and is present in 12-fold molar excess to gelsolin. Both proteins are highly induced during differentiation of the promyelocytic leukemia cell line into macrophages. gCap39 is less abundant in fibroblasts (0.04% total proteins) and is present in equal molar ratio to gelsolin. The two proteins are colocalized in the cytoplasm, but gCap39 is also found in the nucleus while gelsolin is not. Nuclear gCap39 redistributes throughout the cytoplasm during mitosis and is excluded from regions containing chromosomes. Our results demonstrate that gCap39 is a nuclear and cytoplasmic protein which has unique as well as common functions compared with gelsolin.