Molecular evolution of spectrin repeats

Evolution of spectrin function in cytoskeletal and membrane networks

Spectrin is a cytoskeletal protein thought to have descended from an alpha-actinin-like ancestor. It emerged during evolution of animals to promote integration of cells into tissues by assembling signalling and cell adhesion complexes, by enhancing the mechanical stability of membranes and by promoting assembly of specialized membrane domains. Spectrin functions as an (alphabeta([H]))(2) tetramer that cross-links transmembrane proteins, membrane lipids and the actin cytoskeleton, either directly or via adaptor proteins such as ankyrin and 4.1. In the present paper, I review recent findings on the origins and adaptations in this system. (i) The genome of the choanoflagellate Monosiga brevicollis encodes alpha-, beta- and beta(Heavy)-spectrin, indicating that spectrins evolved in the immediate unicellular precursors of animals. (ii) Ankyrin and 4.1 are not encoded in that genome, indicating that spectrin gained function during subsequent animal evolution. (iii) Protein 4.1 gained a spectrin-binding activity in the evolution of vertebrates. (iv) Interaction of chicken or mammal beta-spectrin with PtdInsP(2) can be regulated by differential mRNA splicing, which can eliminate the PH (pleckstrin homology) domain in betaI- or betaII-spectrins; in the case of mammalian betaII-spectrin, the alternative C-terminal region encodes a phosphorylation site that regulates interaction with alpha-spectrin. (v) In mammalian evolution, the single pre-existing alpha-spectrin gene was duplicated, and one of the resulting pair (alphaI) neo-functionalized for rapid make-and-break of tetramers. I hypothesize that the elasticity of mammalian non-nucleated erythrocytes depends on the dynamic rearrangement of spectrin dimers/tetramers under the shearing forces experienced in circulation.

Tracing misalignment and unequal crossing over during evolution of the repetitive region of the SM50 gene in sea urchins

The location of misalignment and unequal crossover involved in concerted evolution of tandemly repetitive sequences is difficult to document owing to the homogeneity of sequences that are subject to this process. However, the repetitive domain of the SM50 gene in sea urchins contains variation, within the gene itself, between alleles, and between species that has allowed us to determine where misalignment and unequal crossing over occurred during evolution of this gene. We have therefore analyzed the SM50 repeat regions in a variety of species to determine where recent changes in repeat numbers have occurred, and from this have deduced the mechanisms that lead to these changes. We next tried to determine whether recent misalignment and unequal crossover has produced allelic variation in current populations of sea urchins. We found SM50 alleles within three species that have different numbers of repeats. This marks the first reported documentation of allelic variation in the number of repeats in the SM50 gene. We also show how a single unequal crossover event could have produced the allelic variation. We have found that substitutions and small deletions in the sequences within the repeats can substantially affect how misalignment occurs, resulting in different patterns of repeats after concerted evolution.

The spectrin cytoskeleton influences the surface expression and activation of human transient receptor potential channel 4 channels

Despite over a decade of research, only recently have the mechanisms governing transient receptor potential channel (TRPC) channel function begun to emerge, with an essential role for accessory proteins in this process. We previously identified a tyrosine phosphorylation event as critical in the plasma membrane translocation and activation of hTRPC4 channels following epidermal growth factor (EGF) receptor activation. To further characterize the signaling events underlying this process, a yeast-two hybrid screen was performed on the C terminus of hTRPC4. The intracellular C-terminal region from proline 686 to leucine 977 was used to screen a human brain cDNA library. Two members of the spectrin family, alphaII- and betaV-spectrin, were identified as binding partners. The interaction of hTRPC4 with alphaII-spectrin and betaV-spectrin was confirmed by glutathione S-transferase pulldown and co-immunoprecipitation experiments. Deletion analysis identified amino acids 730-758 of hTRPC4 as critical for the interaction with this region located within a coiled-coil domain, juxtaposing the Ca(2+)/calmodulin- and IP(3)R-binding region (CIRB domain). This region is deleted in the proposed deltahTRPC4 splice variant form, which failed to undergo both EGF-induced membrane insertion and activation, providing a genetic mechanism for regulating channel activity. We also demonstrate that the exocytotic insertion and activation of hTRPC4 following EGF application is accompanied by dissociation from alphaII-spectrin. Furthermore, depletion of alphaII-spectrin by small interference RNA reduces the basal surface expression of alphahTRPC4 and prevents the enhanced membrane insertion in response to EGF application. Importantly, depletion of alphaII-spectrin did not affect the expression of the delta variant. Taken together, these results demonstrate that a direct interaction between hTRPC4 and the spectrin cytoskeleton is involved in the regulation of hTRPC4 surface expression and activation.

Tandem repetitive D domains of the sperm ligand zonadhesin evolve faster in the paralogue than in the orthologue comparison

Gene duplication is regarded as an important evolutionary mechanism creating genetic and phenotypic novelty. At the same time, the evolutionary mechanisms following gene duplication have been a subject of much debate. Here we analyze the sequence evolution of zonadhesin, a mammalian sperm ligand that binds to the oocyte zona pellucida in a species-specific manner. In pig, rabbit, and primates, precursor zonadhesin comprises, among others, one partial and four complete tandem repetitive D domains. The mouse precursor is distinguished by 20 additional partial D3 domains consisting of 120 amino acids each. This gene structure allows sequence comparison in both paralogues and orthologues. Detailed sequence analysis reveals that D domains evolve faster across paralogues than orthologues. Moreover, at the codon level, partial D3 paralogues of mouse show evidence of positive selection, whereas the corresponding orthologues do not. Individual posttranslational motif patterns and positive selection point to neofunctionalization of partial D3 paralogues of mouse, rather than subfunctionalization. However, as we found additional evidence for homogenization by partial gene conversion, sequence evolution of partial D3 paralogues of mouse might be better described as a combination of divergent and convergent evolution. So far, the divergence at the codon level has outbalanced the convergence at the level of smaller fragments. The probable driving force behind the evolutionary patterns observed is sexual selection. We finally discuss whether the functional determination influences the evolutionary regime acting on sperm ligands and egg receptors, respectively.

Modes of evolution in the protease and kringle domains of the plasminogen-prothrombin family

Phylogenetic analysis of protease domains of the vertebrate plasminogen-prothrombin family revealed two major subfamilies: (1) a subfamily containing macrophage-stimulating protein (MSP), hepatocyte growth factor (HGF), plasminogen, and apolipoprotein(a) (APOA); and (2) a subfamily containing prothrombin, HGF activator, and plasminogen activators. There was evidence that these two subfamilies diverged prior to the divergence of amphibians and amniotes. The phylogeny indicated a close relationship of APOA from the European hedgehog, rhesus monkey, and human with plasminogen. Phylogenetic analysis of repeated kringle domains supported the hypothesis that APOA evolved independently in hedgehog and primates through numerous duplications of different kringle domains of the ancestral plasminogen. Phylogenies of kringle domains revealed two modes of evolution: (1) a conservative mode, whereby duplication of kringle domains occurred prior to cladogenesis and the same kringle structure has been maintained in different lineages (exemplified by plasminogen and prothrombin); and (2) a concerted mode, whereby kringle domains have duplicated since cladogenesis and thus orthologous relationships do not exist between kringles of different lineages (exemplified by APOA).