We have recently shown that heterochromatin protein 1 (HP1) interacts with the nuclear envelope in an acetylationdependent manner. Using purified components and in vitro assays, we now demonstrate that HP1 forms a quaternary complex with the inner nuclear membrane protein LBR and a sub-set of core histones. This complex involves histone H3/H4 oligomers, which mediate binding of LBR to HP1 and crosslink these two proteins that do not interact directly with each other. Consistent with previous observations, HP1 and LBR binding to core histones is strongly inhibited when H3/H4 are modified by recombinant CREB-binding protein, revealing a new mechanism for anchoring domains of under-acetylated chromatin to the inner nuclear membrane.

During mammalian spermiogenesis, histones are replaced by transition proteins, which are in turn replaced by protamines P1 and P2. P1 protamine contains a short arginine/serine-rich (RS) domain that is highly phosphorylated before being deposited into sperm chromatin and almost completely dephosphorylated during sperm maturation. We now demonstrate that, in elongating spermatids, this phosphorylation is required for the temporal association of P1 protamine with lamin B receptor (LBR), an inner nuclear membrane protein that also possesses a stretch of RS dipeptides at its nucleoplasmic NH 2 -terminal domain. Previous studies have shown that the cellular protein p32 also binds tightly to the unmodified RS domain of LBR. Extending those findings, we now present evidence that p32 prevents phosphorylation of LBR and furthermore that dissociation of this protein precedes P1 protamine association. Our data suggest that docking of protamine 1 to the nuclear envelope is an important intermediate step in spermiogenesis and reveal a novel role for SR protein kinases and p32.The development of spermatids into spermatozoa, termed spermiogenesis, is characterized by the replacement of histones by the highly basic, arginine-rich, protamines (1). As a result of this exchange, the nucleosomal-type chromatin is transformed into a smooth fiber and compacted in a volume of about 5% of that of a somatic cell nucleus (2, 3). Although the exchange of chromatin proteins during spermiogenesis has long been known, the molecular mechanisms and the signaling pathways governing the histone to protamine transition have remained obscure.The deposition of protamines on sperm chromatin and the subsequent chromatin condensation appear to be controlled by phosphorylation-dephosphorylation events. Protamines are highly phosphorylated, shortly after their synthesis and before binding to DNA, whereas they become largely dephosphorylated during sperm maturation (4 -8). Phosphorylation of P2 protamine has been shown to be essential, because deletion of the calmodulin-dependent protein kinase Camk4, which phosphorylates P2 protamine, impairs the replacement of transition protein-2 with P2 protamine, resulting in defective spermiogenesis and male sterility (9). On the other hand, all P1 protamines contain short arginine/serine-rich (RS) 1 domains that are efficiently phosphorylated by SRPK1 (SR protein kinase 1) (10), but the physiological significance of this modification is mostly unknown.In this respect, Biggiogera et al. (11) reported that protamines initially appear at the nuclear periphery, implying that the nuclear envelope might play a role in the replacement of transition proteins by protamines during spermiogenesis. Given that RS domains mediate protein-protein interactions (12), we sought to investigate the potential interaction of P1 protamine with the inner nuclear membrane protein lamin B receptor (LBR), which also possesses a repeat of RS dipeptides at its nucleoplasmic NH 2 -terminal domain. In the present study we demonstrate a direct a...

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In this study, glycogen distribution during the development of larval salivary glands in two Drosophila species (D. melanogaster and D. auraria) was examined using the osmium tetroxide±potassium ferrocyanide technique at the ultrastructural level. Glycogen particles are found in big clumps in the salivary gland cells of D. melanogaster during the early third instar, mainly at the basal part of the cells; they are also found as scattered particles in the cytoplasm and very close to the apical plasma membrane during the secretory activity at this developmental stage. During the middle third instar, where the production of`glue' secretory granules begins, glycogen particles are mainly scattered and they are seen near the Golgi complexes and secretory granules. No glycogen particles are seen during the late third instar and secretion of`glue' secretory granules. Re-formation of glycogen begins after spiracle eversion. In the larval salivary gland cells of D. auraria, glycogen particles (scattered and in clumps) are seen in every developmental stage. In the larval fat body cells of both species, many huge clumps of glycogen particles are seen, independent of the developmental stage. These results indicate that glycogen provides an important source of energy during development of Drosophila salivary glands, but different species use glycogen at different metabolic pathways, according to their needs.