In the present study, a novel structural motif that can be represented as a combination of the known [beta][alpha][beta]-unit and [psi]-motif is described and analyzed. In theory, there are four possible combinations of the motifs since each of them can exist in two forms, left-handed and right-handed. For this study, we have selected 140 nonhomologous proteins in which 158 combinations of such types have been found. The combination of the right-handed [psi]-motif and the right-handed [beta][alpha][beta]-unit has been shown to occur most often (87 cases out of 158) and the combination of the left-handed [beta][alpha][beta]-unit and the left-handed [psi-motif does not occur at all. Three novel structural trees in which the commonly occurring combinations are taken as the root structures have been constructed. [PUBLICATION ABSTRACT]
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