IDENTIFICATION OF RB18A, A 205 KDA NEW P53 REGULATORY PROTEIN WHICH SHARES ANTIGENIC AND FUNCTIONAL-PROPERTIES WITH P53

Immunological screening with the anti-p53 moAb, PAb1801 of a cDNA expr ession library, prepared from human B lymphoma cells, led us to identi fy a new human 205 kDa protein called RB18A for 'Recognized By PAb1801 moAntibody'. Immunoblotting or immunoprecipitation of fusion protein or in vitro translated protein, respectively, demonstrated that RB18A protein was recognized by several anti-p53 moAb reacting with the N or C-terminal domains of p53. Full length sequence of RB18B cDNA and com puter analysis demonstrated that despite common antigenic determinants between RB18A and p53 proteins, nucleotide and deduced protein sequen ces did not reveal any significant homologies. RB18A mRNA was detected in all tissues tested except in kidney. In addition, RB18A protein sh ared identical functions with p53 protein: binding to DNA or to p53 an d self-oligomerization. Furthermore, RB18A regulated p53 specific bind ing on his DNA consensus binding site. These functions were associated to the C-terminal domain of RB18A protein and more specifically to th e PAb421 binding site present in this domain. The activation by RB18A of p53 binding on DNA was induced through an unstable interaction betw een both proteins. Altogether, our data demonstrated that RB18A protei n shares antigenic and functional properties with p53 and regulated p5 3 functions.