EVIDENCE FOR STEREOSPECIFIC INOSITOL 1,3,4,5-[H-3]TETRAKISPHOSPHATE BINDING-SITES ON RAT-LIVER NUCLEI - DELINEATING INOSITOL 1,3,4,5-TETRAKISPHOSPHATE INTERACTION IN NUCLEAR CALCIUM SIGNALING PROCESS

H-3-Labeled inositol 1,3,4,5-tetrakisphosphate (IP4) binding sites are observed on nuclei isolated from rat liver and devoid of any microsom al, mitochondrial, or plasma membrane constituents. A pH of about 6.5 is found optimum for maximum [H-3]IP4 specific binding that is sensiti ve to changes in pH. The [H-3]IP4 binding on the nuclei can be disting uished into a high affinity site and a low affinity site. The two bind ing sites are characterized by distinct K(D) and B(max) (1.6 nM versus 57.0 nm K(D); 0.25 pmol/mg protein and 3.7 pmol/mg protein B(max)). I P4 is capable of Ca-45(2+) uptake even in the absence of ATP. The calc ium uptake by nuclei is highly sensitive to IP4 Since it is achieved e ven at 1 nm IP4 concentration. Furthermore, data are documented demons trating that a rapid and transient Ca-45(2+) release by inositol 1,4,5 -trisphosphate (IP3) from the intact nuclei can be reversed by IP4. Th e presence of IPs potentiates the action of IP4 in nuclear calcium reu ptake as attested by the rate of calcium uptake by IP4 in the absence of IP3 (0.16 nmol/s/mg of protein) and in the presence of IPs (4.0 nmo l/s/mg of protein). A novel mechanism of nuclear calcium signaling is proposed where IP4 brings calcium into the nuclei mediated by its spec ific putative binding sites.