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Q: Context: The actin-cross-linking protein spectrin is a prominent component of the membrane cytoskeleton. Spectrin is a tetramer of two antiparallel alphabeta-dimers which share a unique and ancient gene structure. The alpha-spectrin and beta-spectrin genes are composed primarily of tandemly repeated 106-amino-acid segments, each of which forms a triple alpha-helical coiled coil. Both the genes and the repeats themselves are homologous. The two genes are thought to be the result of a gene duplication event, and each gene is the product of duplications of the 106-amino-acid repeats. In this work we compare the process of molecular evolution across the repeated segments of the alpha- and beta-spectrin genes. We find that the alpha-spectrin segments have, for the most part, evolved in a homogeneous fashion, while considerable heterogeneity is found among beta-spectrin segments. Several segments with unique known functions are found to have evolved differently than the others. On the basis of heterogeneity of the evolutionary process, we suggest that at least one repeat has a unique function that has yet to be documented. We also present new statistical methods for comparing the evolutionary process between different regions of DNA sequences.
Question: Alpha-spectrin and beta-spectrin subunits form parallel or antiparallel heterodimers?
A:
antiparallel