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RB18A (TRAP220/DRIP205) is a cofactor of transcription. We herein demonstrated that RB18A downregulated p53 and upregulated MDM2 promoters. These RB18A regulations, not modified by p53wt expression, were inhibited by mutant p53 (p53mut) expression, which directly interacts with RB18A D5 domain. In addition, RB18A via its D4 domain, also interacts directly and specifically with MDM2 protein inhibiting p53mut degradation. Altogether, these mechanisms contribute to maintain a high level of p53mut expression in tumor proliferating cells. Therefore, RB18A plays a central role to control p53wt and p53mut protein content and functions in cells through a loop of regulation, which involves MDM2.

Specific membrane receptors for breakdown products of the third component of human complement, C3, are expressed in several mammalian cells. At present, 5 receptors are described, i.e., C3a, C3b, C3d, iC3b and H altered C3b receptors [I]. Biochemical and molecular analysis of 2 of these, namely C3b and C3d receptors, have been approached. ture medium associated with a 72 000 Mr glycoprotein [9], whereas attempts to purify C3b receptors from its membranes have been unsuccessful [lo]. This is the first report of the solubilization from the membrane of the human B lymphoblastoid cell line, Raji, of a 140 OOOMr glycoprotein carrying a C3b binding activity. Some properties of this membrane component were analyzed.

gp 140, the C3d/EBV receptor (CR2), is a specific marker of human B lymphocytes. Very recent data suggest that CR2 is a membrane site involved in early B cell activation. These properties of CR2 led us to analyze the molecular events associated with gp 140. We analyzed whether in some conditions of B lymphocyte activation, CR2 could be phosphorylated. We have found that when highly enriched peripheral B cells were cultured for 48 h with anti-μ Ab and/or SAC, in order to provide an optimal activating signal, phosphorylation of the CR2 was induced.