Vacuolar system-associated protein-60: a protein characterized from bovine granulosa and luteal cells that is associated with intracellular vesicles and related to human 80K-H and murine beta-glucosidase II

Functional effects of Tribbles homolog 2 in bovine ovarian granulosa cells†

Tribbles homologs (TRIB) 1, 2, and 3 represent atypical members of the serine/threonine kinase superfamily. We previously identified TRIB2 as a differentially expressed gene in granulosa cells (GCs) of bovine preovulatory follicles. The current study aimed to further investigate TRIB2 regulation and study its function in the ovary. GCs were collected from follicles at different developmental stages: small antral follicles (SF), dominant follicles (DF) at day 5 of the estrous cycle, and hCG-induced ovulatory follicles (OFs). RT-qPCR analyses showed greater expression of TRIB2 in GC of DF as compared to OF and a significant downregulation of TRIB2 steady-state mRNA amounts by hCG/LH, starting at 6 h through 24 h post-hCG as compared to 0 h. Specific anti-TRIB2 polyclonal antibodies were generated and western blot analysis confirmed TRIB2 downregulation by hCG at the protein level. In vitro studies showed that FSH stimulates TRIB2 expression in GC. Inhibition of TRIB2 using CRISPR/Cas9 resulted in a significant increase in PCNA expression and an increase in steroidogenic enzyme CYP19A1 expression, while TRIB2 overexpression tended to decrease GC proliferation. TRIB2 inhibition also resulted in a decrease in transcription factors connective tissue growth factor (CTGF) and ankyrin repeat domain-containing protein 1 (ANKRD1) expression, while TRIB2 overexpression increased CTGF and ANKRD1. Additionally, western blot analyses showed reduction in ERK1/2 (MAPK3/1) and p38MAPK (MAPK14) phosphorylation levels following TRIB2 inhibition, while TRIB2 overexpression increased p-ERK1/2 and p-p38MAPK. These results provide evidence that TRIB2 modulates MAPK signaling in GC and that TRIB2 could act as a regulator of GC proliferation and function, which could affect steroidogenesis during follicular development.

Role of advanced glycation end products in cellular signaling

Improvements in health care and lifestyle have led to an elevated lifespan and increased focus on age-associated diseases, such as neurodegeneration, cardiovascular disease, frailty and arteriosclerosis. In all these chronic diseases protein, lipid or nucleic acid modifications are involved, including cross-linked and non-degradable aggregates, such as advanced glycation end products (AGEs). Formation of endogenous or uptake of dietary AGEs can lead to further protein modifications and activation of several inflammatory signaling pathways. This review will give an overview of the most prominent AGE-mediated signaling cascades, AGE receptor interactions, prevention of AGE formation and the impact of AGEs during pathophysiological processes.

Molecular characterization and expression of DERL1 in bovine ovarian follicles and corpora lutea

The endoplasmic reticulum (ER) is a major site of protein synthesis and facilitates the folding and assembly of newly synthesized proteins. Misfolded proteins are retrotranslocated across the ER membrane and destroyed at the proteasome. DERL1 is an important protein involved in the retrotranslocation and degradation of a subset of misfolded proteins from the ER. We characterized a 2617 bp cDNA from bovine granulosa cells that corresponded to bovine DERL1. Two transcripts of 3 and 2.6 kb were detected by Northern blot analysis, and showed variations in expression among tissues. During follicular development, DERL1 expression was greater in day 5 dominant follicles compared to small follicles, ovulatory follicles, or corpus luteum (CL). Within the CL, DERL1 mRNA expression was intermediate in midcycle, and lowest in late cycle as compared to early in the estrous cycle. Western blot analyses demonstrated the presence of DERL1 in the bovine CL at days 5, 11, and 18 of the estrous cycle. Co-immunoprecipitation using luteal tissues showed that DERL1 interacts with class I MHC but not with VIMP or p97 ATPase. The interaction between DERL1 and MHC I suggests that, in the CL, DERL1 may regulate the integrity of MHC I molecules that are transported to the ER membrane. Furthermore, the greater expression of DERL1 mRNA is associated with the active follicular development and early luteal stages, suggesting a role of DERL1 in tissue remodeling events and maintenance of function in reproductive tissues.

Identification of calcium-binding proteins associated with the human sperm plasma membrane

BACKGROUND

The precise composition of the human sperm plasma membrane, the molecular interactions that define domain specific functions, and the regulation of membrane associated proteins during the capacitation process, still remain to be fully understood. Here, we investigated the repertoire of calcium-regulated proteins associated with the human sperm plasma membrane.

METHODS

Surface specific radioiodination was combined with two-dimensional gel electrophoresis, a 45Ca-overlay assay, computer assisted image analysis and mass spectrometry to identify calcium-binding proteins exposed on the human sperm surface.

RESULTS

Nine acidic 45Ca-binding sperm proteins were excised from stained preparative 2D gels and identified by mass spectrometry. Five of the calcium binding proteins; HSPA2 (HSP70-1), HSPA5 (Bip), HYOU1 (ORP150), serum amyloid P-component (SAP) and protein kinase C substrate 80K-H (80K-H) were found to be accessible to Iodo-Bead catalyzed 125I-labelling on the surface of intact human sperm. Agglutination and immunofluorescence analysis confirmed that SAP is situated on the plasma membrane of intact, motile sperm as well as permeabilized cells. Western blot analysis showed increased phosphorylation of human sperm 80K-H protein following in vitro capacitation. This is the first demonstration of the 80K-H protein in a mammalian sperm.

CONCLUSION

The presence of SAP on the surface of mature sperm implies that SAP has a physiological role in reproduction, which is thought to be in the removal of spermatozoa from the female genital tract via phagocytosis. Since 80K-H is a Ca2+-sensor recently implicated in the regulation of both inositol 1,4,5-trisphosphate receptor and transient receptor potential (TRP) cation channel activities, its detection in sperm represents the first direct signaling link between PKC and store-operated calcium channels identified in human sperm.

Exocytosis mechanisms underlying insulin release and glucose uptake: conserved roles for Munc18c and syntaxin 4

Type 2 diabetes has been coined "a two-hit disease," as it involves specific defects of glucose-stimulated insulin secretion from the pancreatic beta cells in addition to defects in peripheral tissue insulin action required for glucose uptake. Both of these processes, insulin secretion and glucose uptake, are mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein core complexes composed of syntaxin, SNAP-23/25, and VAMP proteins. The SNARE core complex is regulated by the Sec1/Munc18 (SM) family of proteins, which selectively bind to their cognate syntaxin isoforms with high affinity. The process of insulin secretion uses multiple Munc18-syntaxin isoform pairs, whereas insulin action in the peripheral tissues appears to use only the Munc18c-syntaxin 4 pair. Importantly, recent reports have linked obesity and Type 2 diabetes in humans with changes in protein levels and single nucleotide polymorphisms (SNPs) of Munc18 and syntaxin isoforms relevant to these exocytotic processes, although the molecular mechanisms underlying the observed phenotypes remain incomplete (5, 104, 144). Given the conservation of these proteins in two seemingly disparate processes and the need to design and implement novel and more effective clinical interventions, it will be vitally important to delineate the mechanisms governing these conserved SNARE-mediated exocytosis events. Thus, we provide here an up-to-date historical review of advancements in defining the roles and molecular mechanisms of Munc18-syntaxin complexes in the pathophysiology of Type 2 diabetes.

Structure of the bovine VASAP-60/PRKCSH gene, functional analysis of the promoter, and gene expression analysis

Vacuolar system-associated protein-60 (VASAP-60) constitutes the bovine ortholog of the human "protein kinase C substrate 80K-H" (PRKCSH or 80K-H). We characterized the bovine VASAP-60/PRKCSH gene structure and promoter, identified cis-acting elements controlling VASAP-60 expression, searched for mRNA splice variants, and analyzed mRNA expression in ovarian follicles. Expression of VASAP-60 mRNA showed a 2.4-fold increase (P<0.0001) in granulosa cells of dominant follicles compared to small follicles (2-4 mm) or ovulatory follicles, and no mRNA splice variant was identified. The bovine VASAP-60 gene encompasses 12.5 kb and is composed of 18 exons and 17 introns. Primer extension analysis revealed a single transcription initiation site, and the promoter lacks a TATA box. Promoter activity assays were performed with a series of deletion constructs in different bovine cell lines (endometrial epithelial glandular, kidney epithelial and aortic endothelial) to identify cis-acting elements. The -53/+16 bp fragment (+1 = transcription start site) conferred minimal promoter activity whereas activator and repressor elements were located in the -200/-53 bp and -653/-200 bp fragments, respectively. Analysis of cis-acting elements in the -200/-53 bp activation domain revealed by gel shift assays and chromatin immunoprecipitation assay that transcription factor YY1 binds to VASAP-60 promoter. This study is the first to report that VASAP-60 is up-regulated in granulosa cells of dominant follicles, to document the primary structure of the bovine VASAP-60 gene and promoter, and to demonstrate that YY1 binds to the VASAP-60 proximal promoter and may act as a positive transcriptional regulator.

Perturbed skeletal muscle insulin signaling in the adult female intrauterine growth-restricted rat

To determine the molecular mechanism(s) linking fetal adaptations in intrauterine growth restriction (IUGR) to adult maladaptations of type 2 diabetes mellitus, we investigated the effect of prenatal seminutrient restriction, modified by early postnatal ad libitum access to nutrients (CM/SP) or seminutrient restriction (SM/SP), vs. early postnatal seminutrient restriction alone (SM/CP) or control nutrition (CM/CP) on the skeletal muscle postreceptor insulin-signaling pathway in the adult offspring. The altered in utero hormonal/metabolic milieu was associated with no change in basal total IRS-1, p85, and p110beta subunits of PI 3-kinase, PKCtheta, and PKCzeta concentrations but an increase in basal IRS-2 (P < 0.05) only in the CM/SP group and an increase in basal phospho (p)-PDK-1 (P < 0.05), p-Akt (P < 0.05), and p-PKCzeta (P < 0.05) concentrations in the CM/SP and SM/SP groups. Insulin-stimulated increases in p-PDK-1 (P < 0.05) and p-Akt (P < 0.0007), with no increase in p-PKCzeta, were seen in both CM/SP and SM/SP groups. SHP2 (P < 0.03) and PTP1B (P < 0.03) increased only in SM/SP with no change in PTEN in CM/SP and SM/SP groups. Aberrations in kinase and phosphatase moieties in the adult IUGR offspring were initiated in utero but further sculpted by the early postnatal nutritional state. Although the CM/SP group demonstrated enhanced kinase activation, the SM/SP group revealed an added increase in phosphatase concentrations with the net result of heightened basal insulin sensitivity in both groups. The inability to further respond to exogenous insulin was due to the key molecular distal roadblock consisting of resistance to phosphorylate and activate PKCzeta necessary for GLUT4 translocation. This protective adaptation may become maladaptive and serve as a forerunner for gestational and type 2 diabetes mellitus.

Identification of 80K-H as a protein involved in GLUT4 vesicle trafficking

PKCzeta (protein kinase Czeta) is a serine/threonine protein kinase controlled by insulin, various growth factors and phosphoinositide 3-kinase. It has been implicated in controlling glucose transport in response to insulin by the translocation of GLUT4-(glucose transporter 4) containing vesicles to the plasma membrane in stimulated cells. How PKCzeta modulates GLUT4 vesicle trafficking remains unknown. A yeast two-hybrid screen using full-length human PKCzeta identified 80K-H protein as an interactor with PKCzeta. GST (glutathione S-transferase) pull-down assays with GST-tagged 80K-H constructs confirmed the interaction and showed that the N-terminal portion of 80K-H was not required for the interaction. Immunoprecipitates of endogenous PKCzeta from Cho cells, 3T3-L1 adipocytes or L6 myotubes contained endogenous 80K-H, demonstrating a physiological interaction. Insulin stimulation enhanced the association 3-5-fold. Immunoprecipitates of endogenous 80K-H contained endogenous munc18c and immunoprecipitates of endogenous munc18c contained endogenous PKCzeta, with insulin markedly increasing the amount of co-immunoprecipitated protein in each case. These results show that insulin triggers interactions in vivo between PKCzeta, 80K-H and munc18c. Overexpression of 80K-H constructs mimicked the action of insulin in stimulating both glucose uptake and translocation of Myc-tagged GLUT4 in Cho cells, with the level of effect proportional to the ability of the constructs to associate with munc18c. These results identify 80K-H as a new player involved in GLUT4 vesicle transport and identify a link between a kinase involved in the insulin signalling cascade, PKCzeta, and a known component of the GLUT4 vesicle trafficking pathway, munc18c. The results suggest a model whereby insulin triggers the formation of a PKCzeta-80K-H-munc18c complex that enhances GLUT4 translocation to the plasma membrane.

80K-H as a New Ca2+ Sensor Regulating the Activity of the Epithelial Ca2+ Channel Transient Receptor Potential Cation Channel V5 (TRPV5)

The epithelial Ca(2+) channel transient receptor potential cation channel V5 (TRPV5) constitutes the apical Ca(2+) entry pathway in the process of active Ca(2+) reabsorption. Ca(2+) influx through TRPV5 is tightly controlled by modulators of Ca(2+) homeostasis, including 1,25-dihydroxyvitamin D(3) and dietary Ca(2+). However, little is known about intracellular proteins that interact with TRPV5 and directly regulate the activation of this channel. By the use of cDNA microarrays, the present study identified 80K-H as the first protein involved in the Ca(2+)-dependent control of the epithelial Ca(2+) channel TRPV5. 80K-H was initially identified as a protein kinase C substrate, but its biological function remains to be established. We demonstrated a specific interaction between 80K-H and TRPV5, co-localization of both proteins in the kidney, and similar transcriptional regulation by 1,25-dihydroxyvitamin D(3) and dietary Ca(2+). Furthermore, 80K-H directly bound Ca(2+), and inactivation of its two EF-hand structures totally abolished Ca(2+) binding. Electrophysiological studies using 80K-H mutants showed that three domains of 80K-H (the two EF-hand structures, the highly acidic glutamic stretch, and the His-Asp-Glu-Leu sequence) are critical determinants for TRPV5 activity. Importantly, inactivation of the EF-hand pair reduced the TRPV5-mediated Ca(2+) current and increased the TRPV5 sensitivity to intracellular Ca(2+), accelerating the feedback inhibition of the channel. None of the 80K-H mutants altered the TRPV5 plasma membrane localization nor the association of 80K-H with TRPV5, suggesting that 80K-H has a direct effect on TRPV5 activity. In conclusion, we report a novel function for 80K-H as a Ca(2+) sensor controlling TRPV5 channel activity.

Human chorionic gonadotropin-dependent regulation of 17beta-hydroxysteroid dehydrogenase type 4 in preovulatory follicles and its potential role in follicular luteinization

17Beta-hydroxysteroid dehydrogenase type 4 (17betaHSD4) has a unique multidomain structure, with one domain involved in 17beta-estradiol inactivation. The objective of the study was to investigate the regulation of 17betaHSD4 during human chorionic gonadotropin (hCG)-induced ovulation/luteinization. The equine 17betaHSD4 cDNA was cloned and was shown to encode a 735-amino acid protein that is highly conserved (81-87% identity) compared with other mammalian orthologs. RT-PCR/Southern blot analyses were performed to study the regulation of 17betaHSD4 transcripts in equine preovulatory follicles isolated between 0-39 h after hCG treatment. Results showed the presence of basal 17betaHSD4 mRNA expression before hCG treatment, but an increase was observed in follicles obtained 24 h after hCG (P < 0.05). Analyses of isolated preparations of granulosa and theca interna cells identified basal mRNA expression in both layers, but granulosa cells appeared as the predominant site of follicular 17betaHSD4 mRNA induction. A specific polyclonal antibody was raised against a fragment of the equine protein and used to study regulation of the 17betaHSD4 protein. Immunoblots showed an increase in full-length 17betaHSD4 protein in follicles 24 h after hCG (P < 0.05), in keeping with mRNA results. Immunohistochemical data confirmed the induction of the enzyme in follicular cells after hCG treatment. Collectively, these results demonstrate that the gonadotropin-dependent induction of follicular luteinization is accompanied by an increase in 17betaHSD4 expression. Considering the estrogen-inactivating function of 17betaHSD4, its regulated expression in luteinizing preovulatory follicles appears as a potential complementary mechanism to reduce circulating levels of 17beta-estradiol after the LH surge.

Immunolocalization of vacuolar system-associated protein-60 (VASAP-60)

We have characterized the localization of the protein termed VASAP-60 in different bovine tissues and cell lines, and have investigated if VASAP-60 interacts with other proteins. Monospecific polyclonal antibodies were raised against distinct fragments of VASAP-60: NH(2) (V(22) to Q(234)), central (A(246) to S(418)), and COOH (L(416) to L(533)). These three antibodies recognized an 88-kDa protein in immunoblotting analysis. The calculated Mr of VASAP-60 derived from its cDNA (60.1 kDa) was significantly lower than its Mr estimated by SDS-PAGE, and this was mainly attributed to the glutamic acid- and aspartic acid-rich composition of its central region (A(246) to S(418)). A 58-kDa proteolytically processed form of VASAP-60 was also identified. Immunocytochemistry demonstrated that VASAP-60 is found predominantly in the perinuclear region, colocalized with calnexin in the endoplasmic reticulum (ER), and partially colocalized with the endocytic marker DAMP. Immunohistochemical localization of VASAP-60 also demonstrated its presence within specialized vesicular structures not related to the ER. Immunoprecipitation using extracts prepared from S(35)Met/Cys metabolically labeled cells demonstrates that VASAP-60 interacts with 116-, 48.5-, and 26.5-kDa proteins. Therefore, VASAP-60 was found to be more widely distributed in the vacuolar system than anticipated, suggesting that VASAP-60 may function in intracellular transport events, rather than being an exclusive component of the quality control mechanism of newly synthesized proteins as thought previously.

Mutations in PRKCSH cause isolated autosomal dominant polycystic liver disease

Autosomal dominant polycystic liver disease (ADPLD) is a distinct clinical and genetic entity that can occur independently from autosomal dominant polycystic kidney disease (ADPKD). We previously studied two large kindreds and reported localization of a gene for ADPLD to an approximately 8-Mb region, flanked by markers D19S586/D19S583 and D19S593/D19S579, on chromosome 19p13.2-13.1. Expansion of these kindreds and identification of an additional family allowed us to define flanking markers CA267 and CA048 in an approximately 3-Mb region containing >70 candidate genes. We used a combination of denaturing high-performance liquid chromatography (DHPLC) heteroduplex analysis and direct sequencing to screen a panel of 15 unrelated affected individuals for mutations in genes from this interval. We found sequence variations in a known gene, PRKCSH, that were not observed in control individuals, that segregated with the disease haplotype, and that were predicted to be chain-terminating mutations. In contrast to PKD1, PKD2, and PKHD1, PRKCSH encodes a previously described human protein termed "protein kinase C substrate 80K-H" or "noncatalytic beta-subunit of glucosidase II." This protein is highly conserved, is expressed in all tissues tested, and contains a leader sequence, an LDLa domain, two EF-hand domains, and a conserved C-terminal HDEL sequence. Its function may be dependent on calcium binding, and its putative actions include the regulation of N-glycosylation of proteins and signal transduction via fibroblast growth-factor receptor. In light of the focal nature of liver cysts in ADPLD, the apparent loss-of-function mutations in PRKCSH, and the two-hit mechanism operational in dominant polycystic kidney disease, ADPLD may also occur by a two-hit mechanism.

Serine protease inhibitor-E2 (SERPINE2) is differentially expressed in granulosa cells of dominant follicle in cattle

The objective was to analyze gene expression in bovine granulosa cells of the dominant follicle by mRNA differential display. Total RNA was extracted from granulosa cells of <or=4 mm follicles, day 5 (D5) dominant follicles, and hCG-induced preovulatory follicles. A differentially expressed cDNA observed in the dominant follicle group was used to screen a granulosa cell cDNA library, which resulted in the cloning of a 2,096 bp cDNA. Amino acid comparison showed identity level of 91.4, 83.9, and 83.1% when compared to human, rat, and mouse serine protease inhibitor E2, SERPINE2, also called Glia-derived nexin or protease Nexin-1. A single transcript of 2.4 kb was shown to be differentially expressed in different bovine tissues. Immunoblotting with a specific antibody raised against a fragment of SERPINE2 (S(12)-R(196)) showed that SERPINE2 migrated at 47.5 kDa in support of glycosylation. Primordial, primary, and secondary pre-antral follicles showed immunostaining associated with granulosa cells and oocytes, and strong labeling in large antral follicles was located with granulosa cells and follicular fluid. Heterogeneity of SERPINE2 labeling was observed in CL. Semi-quantitative real-time fluorescent RT-PCR showed a six-fold increase (P = 0.0002) in mRNA level of SERPINE2 in granulosa cells of D5 dominant follicle compared to granulosa cells collected from the <or=4 mm or preovulatory hCG-induced follicles. This report demonstrates that SERPINE2 mRNA is regulated in a spatio-temporal pattern with highest levels in granulosa cells of growing dominant bovine follicles, and support the hypothesis that a high expression of SERPINE2 may contribute to follicular growth whereas a decrease following hCG injection may contribute to ovulation.

Checkpoints for vesicular traffic?

During interphase the transport of material between different intracellular organelles requires accurate regulation of fusiogenic domains. Recent studies on hepatic endosomes indicated that compartmentalized Cdk2 – cyclin E complexes act by braking fusion events. These Cdk2 complexes integrate tyrosine phosphorylation and dephosphory lation inputs, resulting in the control of the number of rounds of fusion at discrete domains. This leads to changes in the intracellular location of internalized receptors and ultimately their biological response.Key words: vesicular traffic, Cdk2, receptors tyrosine kinases.

Molecular cloning and induction of bovine prostaglandin E synthase by gonadotropins in ovarian follicles prior to ovulation in vivo

Prostaglandin E(2) (PGE(2)) is thought to be an ultimate prostaglandin effector during the ovulatory process, and the objectives of this study were to clone bovine PGE synthase (PGES) and to characterize its regulation by gonadotropins in preovulatory follicles in vivo. The bovine PGES complementary DNA (cDNA) was shown to contain a 5'-untranslated region of eight base pairs (bp), an open reading frame of 462 bp and a 3'-untranslated region of 406 bp. The putative bovine PGES open reading frame encodes a 153-amino acid protein that is 85, 78, and 78% identical to the human, rat, and mouse PGES homologs, respectively. The regulation of PGES during ovulation was studied using three different models in vivo: 1) human chorionic gonadotropin (hCG)-induced ovulation during a normal estrous cycle; 2) hCG-induced ovulation following ovarian hyperstimulation; and 3) spontaneous ovulation during natural estrus. Results from semi-quantitative reverse transcription-polymerase chain reaction/Southern blotting analyses showed that the hCG/luteinizing hormone surge caused a significant increase in PGES mRNA. Levels of PGES transcripts were low or undetectable prior to hCG/luteinizing hormone but increased markedly 18-24 h after hCG in models 1 and 2, and 18-24 h after the onset of natural estrus in model 3 (p < 0.05). Analyses on isolated preparations of granulosa and theca interna cells indicated that the granulosa cell layer was the predominant site of follicular PGES expression. The regulation of the protein was studied in the same models using a specific antibody raised against a fragment of bovine protein (Delta PGES; from Glu(49) to Val(146)). Results from immunoblots showed an induction of bovine PGES (M(r) = 17,000) 18-24 h after hCG treatment or onset of estrus (p < 0.05). The protein was detected in extracts of granulosa cells but not in theca interna. Collectively, these results demonstrate that the ovulatory process is associated with a gonadotropin-dependent induction of PGES in granulosa cells of ovarian follicles in vivo, thus establishing for the first time the regulation of the enzyme in a physiological context.

Characterization of Cdk2-cyclin E complexes in plasma membrane and endosomes of liver parenchyma. Insulin-dependent regulation

Rat liver parenchyma Golgi/endosomes fractions harbor a tyrosine-phosphorylated 34-kDa protein. Screening of Golgi, endosomes (ENs), plasmalemma (PM), and cytosolic (Cyt) fractions revealed the presence of the mitotic kinase Cdk2 in ENs, PM, and Cyt. The fluid phase endocytic marker horseradish peroxidase gained access to the endosomal Cdk2, confirming its localization. Cdk2 was shown to be associated to cyclin E and was active in ENs and PM fractions. The administration of a single dose of insulin (1.5 microgram/100 g, body weight) induced a time-dependent activation of the insulin receptor kinase in these structures. Insulin receptor-kinase activation was followed by the inhibition of immunoprecipitated Cdk2-cyclin E kinase activity in PM and the progressive disappearance of cyclin E. In marked contrast, no such effect was observed in ENs. The injection of a phosphotyrosyl phosphatase inhibitor (bpV(phen)) increased the levels of cyclin E in ENs and PM. A massive recruitment of p27(kip1) was observed in the Cdk2-cyclin E complexes isolated from PM and Cyt but not from ENs. In vitro, Cdk2-cyclin E complexes have the capacity to inhibit the formation of hybrid structures containing horseradish peroxidase and radioiodinated epidermal growth factor. Therefore, in the PM and ENs of adult rat liver, an active and regulated pool of the mitotic kinase Cdk2-cyclin E and some yet to be defined effectors are present. Cdk2 may contribute to the modulation of transport events and/or maintenance of the topology of endocytic elements.