Specific interaction with the nuclear transporter importin alpha2 can modulate paraspeckle protein 1 delivery to nuclear paraspeckles

Importin (IMP) superfamily members mediate regulated nucleocytoplasmic transport, which is central to key cellular processes. Although individual IMPalpha proteins exhibit dynamic synthesis and subcellular localization during cellular differentiation, including during spermatogenesis, little is known of how this affects cell fate. To investigate how IMPalphas control cellular development, we conducted a yeast-two-hybrid screen for IMPalpha2 cargoes in embryonic day 12.5 mouse testis, a site of peak IMPalpha2 expression coincident with germline masculization. We identified paraspeckle protein 1 (PSPC1), the original defining component of nuclear paraspeckles, as an IMPalpha2 binding partner. PSPC1-IMPalpha2 binding in testis was confirmed in immunoprecipitations and pull-downs, and an ELISA-based assay demonstrated direct, high-affinity PSPC1 binding to either IMPalpha2/IMPbeta1 or IMPalpha6/IMPbeta1. Co-expression of full length PSPC1 and IMPalpha2 in HeLa cells yielded increased PSPC1 localization in nuclear paraspeckles. High throughput image analysis of >3500 cells indicated IMPalpha2 levels can directly determine PSPC1-positive nuclear speckle numbers and size; a transport-deficient IMPalpha2 isoform or siRNA knockdown of IMPalpha2 both reduced endogenous PSPC1 accumulation in speckles. This first validation of an IMPalpha2 nuclear import cargo in fetal testis provides novel evidence that PSPC1 delivery to paraspeckles, and consequently paraspeckle function, may be controlled by modulated synthesis of specific IMPs.