The gene mtsl, which is expressed specifically in metastatic cells, was isolated by molecular cloning coupled with differential DNA reassociation. Transcription of mtsl was found not only in tumor cells, but also in normal cells; homologous RNA was detected only in spleen, thymus, bone marrow, and blood lymphocytes. DNA sequencing of mtsl revealed an open reading frame containing information for a peptide of 101 amino acids, and the amino acid sequence suggested that the mtsl protein was identical to the previously isolated Ca2+-binding mouse protein (Jackson-Grusby et al. 1987; Goto et al. 1988). Thus, the mtsl protein is a member of the calcium-modulated protein family, and our data indicate that mtsl is involved in regulating the metastatic behavior of tumor cells.

Intrinsically disordered regions occur frequently in proteins and are characterized by a lack of a well-defined three-dimensional structure. Although these regions do not show a higher order of structural organization, they are known to be functionally important. Disordered regions are rapidly evolving, largely attributed to relaxed purifying selection and an increased role of genetic drift. It has also been suggested that positive selection might contribute to their rapid diversification. However, for our own species, it is currently unknown whether positive selection has played a role during the evolution of these protein regions. Here, we address this question by investigating the evolutionary pattern of more than 6600 human proteins with intrinsically disordered regions and their ordered counterparts. Our comparative approach with data from more than 90 mammalian genomes uses a priori knowledge of disordered protein regions, and we show that this increases the power to detect positive selection by an order of magnitude. We can confirm that human intrinsically disordered regions evolve more rapidly, not only within humans but also across the entire mammalian phylogeny. They have, however, experienced substantial evolutionary constraint, hinting at their fundamental functional importance. We find compelling evidence that disordered protein regions are frequent targets of positive selection and estimate that the relative rate of adaptive substitutions differs fourfold between disordered and ordered protein regions in humans. Our results suggest that disordered protein regions are important targets of genetic innovation and that the contribution of positive selection in these regions is more pronounced than in other protein parts.