Human Spindly is required for kinetochore localization of cytoplasmic dynein, which is essential for poleward movement of chromosomes and for kinetochore protein streaming. In addition, Spindly controls the activity and kinetochore abundance of the RZZ complex, which contributes to microtubule attachment and mitotic checkpoint activity.

Human chromokinesins hKID and KIF4A contribute to proper attachment of chromosomes by controlling the positioning of the chromosome arms and microtubule dynamics, respectively.

Cyclin-dependent kinase 16 (CDK16, PCTK1) is a poorly characterized protein kinase, highly expressed in the testis and the brain. Here, we report that CDK16 is activated by membrane-associated cyclin Y (CCNY). Treatment of transfected human cells with the protein kinase A (PKA) activator forskolin blocked, while kinase inhibition promoted, CCNY-dependent targeting of CDK16-green fluorescent protein (GFP) to the cell membrane. CCNY binding to CDK16 required a region upstream of the kinase domain and was found to be inhibited by phosphorylation of serine 153, a potential PKA phosphorylation site. Thus, in contrast to other CDKs, CDK16 is regulated by phosphorylation-controlled cyclin binding. CDK16 isolated from murine testis was unphosphorylated, interacted with CCNY, and exhibited kinase activity. To investigate the function of CDK16 in vivo, we established a conditional knockout allele. Mice lacking CDK16 developed normally, but male mice were infertile. Spermatozoa isolated from their epididymis displayed thinning and elongation of the annulus region, adopted a bent shape, and showed impaired motility. Moreover, CDK16-deficient spermatozoa had malformed heads and excess residual cytoplasm, suggesting a role of CDK16 in spermiation. Thus, CDK16 is a membrane-targeted CDK essential for spermatogenesis.

The anaphase-promoting complex/cyclosome (APC/C) is essential for progression through mitosis. At anaphase onset, the APC/C requires the activator protein CDC20 to target securin and cyclin B1 for proteasome-dependent degradation, but then depends on the CDC20-related protein FZR1 (also known as CDH1) to remain active until the onset of the next S phase. To investigate the role of FZR1 in mammalian cells, we used RNAi in human cell lines and conditional gene targeting in mouse embryonic fibroblasts. In neither case was FZR1 required for exit from mitosis, but in cells lacking FZR1, the G1 phase was shortened and the S phase was prolonged. In several normal and transformed human cell lines, loss of FZR1 function induced DNA-damage responses and impaired proliferation independently of the p53 status. Constitutive knockdown of p53 in U2OS cells with inducible FZR1 siRNA also failed to restore their proliferative capacity. Thus, the proliferation defects are a direct consequence of the genetic damage inflicted by loss of FZR1 function and are largely independent of p53. In summary, mammalian FZR1 is not required for the completion of mitosis, but is an important regulator of G1 phase and is required for efficient DNA replication in human and mouse somatic cells.

Suppressor of cytokine signaling (SOCS) proteins play a pivotal role in the development and progression of various cancers. We have previously shown that SOCS-3 is expressed in prostate cancer, and its expression is inversely correlated with activation of signal transducer and activator of transcription factor 3. We hypothesized that SOCS-1, if expressed in prostate cancer cells, has a growth-regulatory role in this malignancy. The presence of both SOCS-1 mRNA and protein was detected in all tested cell lines. To assess SOCS-1 expression levels in vivo, we analyzed tissue microarrays and found a high percentage of positive cells in both prostate intraepithelial neoplasias and cancers. SOCS-1 expression levels decreased in samples taken from patients undergoing hormonal therapy but increased in specimens from patients who failed therapy. In LNCaP-interleukin-6- prostate cancer cells, SOCS-1 was up-regulated by interleukin-6 and in PC3-AR cells by androgens; such up-regulation was also found to significantly impair cell proliferation. To corroborate these findings, we used a specific small interfering RNA against SOCS-1 and blocked expression of the protein. Down-regulation of SOCS-1 expression caused a potent growth stimulation of PC3, DU-145, and LNCaP-interleukin-6- cells that was associated with the increased expression levels of cyclins D1 and E as well as cyclin-dependent kinases 2 and 4. In summary, we show that SOCS-1 is expressed in prostate cancer both in vitro and in vivo and acts as a negative growth regulator.