ABSTRACT
Caveolae are an abundant feature of mammalian cells. Integral membrane proteins called caveolins drive the formation of caveolae but the precise mechanisms underlying caveola formation, and the origin of caveolae and caveolins during evolution, are unknown. Systematic evolutionary analysis shows conservation of genes encoding caveolins in metazoans. We provide evidence for extensive and ancient, local and genomic gene duplication, and classify distinct caveolin gene families. Vertebrate caveolin-1 and caveolin-3 isoforms, as well as an invertebrate (Apis mellifera, honeybee) caveolin, all form morphologically identical caveolae in caveolin-1-null mouse cells, demonstrating that caveola formation is a conserved feature of evolutionarily distant caveolins. However, coexpression of flotillin-1 and flotillin-2 did not cause caveola biogenesis in this system. In contrast to the other tested caveolins, C. elegans caveolin is efficiently transported to the plasma membrane but does not generate caveolae, providing evidence of diversity of function in the caveolin gene family. Using C. elegans caveolin as a template to generate hybrid caveolin constructs we now define domains of caveolin required for caveolae biogenesis. These studies lead to a model for caveola formation and novel insights into the evolution of caveolin function.
Subject(s)
Caenorhabditis elegans , Caveolae/physiology , Caveolins/metabolism , Membrane Proteins/metabolism , Protein Isoforms/metabolism , Amino Acid Sequence , Animals , Caveolae/ultrastructure , Caveolins/deficiency , Caveolins/genetics , Cell Line , Fibroblasts/cytology , Fibroblasts/metabolism , Humans , Membrane Proteins/genetics , Mice , Mice, Knockout , Microscopy, Confocal , Molecular Sequence Data , Organelle Biogenesis , Phylogeny , Protein Isoforms/genetics , Protein Sorting Signals , Protein Transport/genetics , Rats , Recombinant Fusion Proteins/genetics , Recombinant Fusion Proteins/metabolism , Sequence Deletion , TransfectionABSTRACT
In humans, there are two isoforms each of clathrin heavy chain (CHC17 and CHC22) and light chain (LCa and LCb) subunits, all encoded by separate genes. CHC17 forms the ubiquitous clathrin-coated vesicles that mediate membrane traffic. CHC22 is implicated in specialized membrane organization in skeletal muscle. CHC17 is bound and regulated by LCa and LCb, whereas CHC22 does not functionally interact with either light chain. The imbalanced interactions between clathrin subunit isoforms suggest a distinct evolutionary history for each isoform pair. Phylogenetic and sequence analysis placed both heavy and light chain gene duplications during chordate evolution, 510-600 million years ago. Genes encoding CHC22 orthologues were found in several vertebrate species, with only a pseudogene present in mice. Multiple paralogons surrounding the CHC genes (CLTC and CLTD) were identified, evidence that genomic or large-scale gene duplication produced the two CHC isoforms. In contrast, clathrin light chain genes (CLTA and CLTB) apparently arose by localized duplication, within 1-11 million years of CHC gene duplication. Analysis of sequence divergence patterns suggested that structural features of the CHCs were maintained after gene duplication, but new interactions with regulatory proteins evolved for the CHC22 isoform. Thus, independent mechanisms of gene duplication expanded clathrin functions, concomitant with development of neuromuscular sophistication in chordates.
Subject(s)
Chordata , Clathrin Heavy Chains/genetics , Clathrin Light Chains/genetics , Evolution, Molecular , Gene Duplication , Phylogeny , Animals , Bayes Theorem , Chromosome Mapping , Clathrin Heavy Chains/metabolism , Clathrin Light Chains/metabolism , Computational Biology , Genetic Variation , Humans , Models, Genetic , Protein IsoformsABSTRACT
The clathrin triskelion self-assembles into a polyhedral coat surrounding membrane vesicles that sort receptor cargo to the endocytic pathway. A triskelion comprises three clathrin heavy chains joined at their C-termini, extending into proximal and distal leg segments ending in a globular N-terminal domain. In the clathrin coat, leg segments entwine into parallel and anti-parallel interactions. Here we define the contributions of segmental interactions to the clathrin assembly reaction and measure the strength of their interactions. Proximal and distal leg segments were found to lack sufficient affinity to form stable homo- or heterodimers under assembly conditions. However, chimeric constructs of proximal or distal leg segments, trimerized by replacement of the clathrin trimerization domain with that of the invariant chain protein, were able to self-assemble in reversible reactions. Thus clathrin assembly occurs because weak leg segment affinities are coordinated through trimerization, sharing a dependence on multiple weak interactions with other biopolymers. Such polymerization is sensitive to small environmental changes and is therefore compatible with cellular regulation of assembly, disassembly and curvature during formation of clathrin-coated vesicles.
