CHARACTERIZATION OF FMR1 PROTEINS ISOLATED FROM DIFFERENT TISSUES

FMR1 protein expression was studied in different tissues, In human, mo nkey and murine tissues, high molecular mass FMR1 proteins (67-80 kDa) are found, as shown in lymphoblastoid cell lines. The identity of the se proteins was confirmed by their absence in tissues from patients wi th the fragile X syndrome and a FMR1 knock-out mouse, An Ile367Asn sub stitution in the FMR1 protein did not alter the translation, processin g and localization of FMR1 proteins in lymphoblastoid cells from a pat ient carrying this mutation, All the high molecular mass FMR1 proteins isolated from normal lymphoblastoid cells and cells from the patient with the Ile367Asn substitution were able to bind RNA, However, the FM R1 proteins of the patient had reduced affinity for RNA binding at hig h salt concentrations, In some human, monkey and murine tissues low mo lecular mass FMR1 proteins (39-41 kDa) were found, which had the same N terminus as the 67-90 kDa isoforms, but differ in their C terminus a nd are therefore most likely the result of carboxy-terminal proteolyti c cleavage, These low molecular mass FMR1 proteins did not bind RNA, i n contrast with the high molecular mass FMR1 proteins; The significanc e of these low molecular mass' proteins remains to be studied.