p19

P19 is a label used for several distinct proteins in biology rather than a single molecule. In common usage, it refers to two widely cited entities: a viral RNA silencing suppressor named P19, and the mouse tumor suppressor product p19Arf (often written p19Arf). The two proteins are unrelated in sequence, structure, and function, and the shared name reflects independent historical naming choices rather than common ancestry.

P19 protein from Carnation Italian ringspot virus (CIRV) is a 19-kilodalton RNA silencing suppressor produced by tombusviruses. It binds short double-stranded RNA molecules, particularly 21- to 25-nucleotide small interfering RNAs, in a manner that sequesters them and prevents their incorporation into the RNA-induced silencing complex (RISC). By blocking RNA silencing, P19 facilitates viral replication and spread in host plants. P19 has been widely used as a molecular tool to suppress gene silencing in plant biotechnology and to study RNA interference mechanisms. The protein typically forms a homodimer, creating a binding interface for the siRNA duplexes.

p19Arf is the mouse-specific product of the Cdkn2a locus, also called Arf. It functions as a tumor suppressor by antagonizing MDM2, thereby stabilizing p53 and promoting cell cycle arrest and senescence. Loss of p19Arf accelerates tumor development in mice, while its activation contributes to tumor suppression. In humans, the functional equivalent is p14ARF (not p19Arf), which performs a similar role in the p53 pathway but differs in sequence and gene organization.