[Role of connections in the formation of protein structures, containing 4-helical segments].

The packing of alpha-helices in protein molecules and domains consisting of four consecutive alpha-helices in primary structures in analyzed. Such 4-alpha-helical complexes can have two "mirror-symmetrical" structures. Analysis is hydrophobic interactions, hydrogen and salt bonds is insufficient for an unambiguous choice of one of the two structures. It is shown in the present paper that severe sterical restrictions are superimposed on the packing of alpha-helices by interhelical regions, or simply connections. Thus, if two alpha-helices are joined by a short connection from two peptide units, this unambiguously sets the packing of these helices within the 4 alpha-helical complex. An unambiguous packing of the entire complex is achieved if the short connection joins the second and third alpha-helices or there are two short connections between the first and second and also the third and fourth alpha-helices. If the short connection joins only the first and second helices, then the length of the connection between the second and third helices should not exceed three peptide units for an unambiguous packing of the 4 alpha-helical complex.