Looking for the functions of RNA granules in ALK-transformed cells

Abstract

Numerous cytoplasmic foci containing mRNA s and their associated proteins have been described in mammalian somatic and germ cells. The best studied examples are given by the processing bodies (PBs) that are present in all cell types, and the stress granules (SGs) that are transiently formed following stress stimuli. Those foci are non-membranous dynamic structures that, through the continuous exchange of their content with the cytoplasm, are believed to control mRNA storage, translation and degradation. However, due in part to the fact that their composition has not been fully characterized, their relevance to mRNA regulation and cell survival remains a matter of debate. In a recent study, we described new cytoplasmic foci that form specifically in transformed cells expressing the constitutively active ALK tyrosine kinase. Those granules, further called AGs for ALK granules, contain polyadenylated mRNAs but are distinct from PBs and SGs. Using a method based on sucrose density gradient fractionation, we further purified AGs and identified their mRNA content. We discuss our findings in relation to other granules containing untranslated mRNAs and speculate on the possible contribution of AGs to the oncogenic properties of ALK-expressing cells.