THE ADDITION OF 9 RESIDUES AT THE C-TERMINUS OF HUMAN PROLACTIN DRASTICALLY ALTERS ITS BIOLOGICAL PROPERTIES

We have added nine extra residues to the C-terminal of human prolactin and analysed the effect of this mutation on the ability of the hormon e to bind to its lactogenic receptor and to induce Nb2 cell division. Both properties are markedly affected when compared to the natural 23- kDa human prolactin. Since no alteration of the global protein folding was detected either by circular dichroism or by infrared spectroscopy , the decrease in biological potency can be exclusively attributed to an effect of the nine additional residues on their near environment. F rom infrared analysis and secondary structure prediction, the elongate d tail is assumed to be involved in a beta-sheet with a few residues i nitially belonging to the fourth helix. Moreover, from die X-ray struc tures of porcine and human growth hormones, two proteins homologous to prolactins, the nine extra residues are likely to fold within a conca ve pocket delimited by helices 1 and 4, and die second half of the loo p connecting helices 1 and 2 (loop 1). Thereby, we suggest that the ad ditional residues prevent some residues belonging to this pocket from interacting with the lactogenic receptor. This is in perfect agreement with our earlier proposal that the binding site of prolactin to the l actogenic receptor is homologous to that of growth hormone to the soma togenic receptor, i.e. essentially composed of residues belonging to t his concave pocket.