The protein tyrosine kinases of the sea urchin Anthocidaris crassispina

Bioinformatic search of plant microtubule-and cell cycle related serine-threonine protein kinases

A bioinformatic search was carried for plant homologues of human serine-threonine protein kinases involved in regulation of cell division and microtubule protein phosphorylation (SLK, PAK6, PAK7, MARK1, MAST2, TTBK1, TTBK2, AURKA, PLK1, PLK4 and PASK). A number of SLK, MAST2 and AURKA plant homologues were identified. The closest identified homologue of human AURKA kinase was a protein of unknown function, A7PY12/GSVIVT00026259001 from Vitis vinifera (herein named as "STALK", Serine-Threonine Aurora-Like Kinase). Analysis of STALK's three-dimensional structure confirmed its relationship to the subgroup of AURKA-like protein kinases.

Expression of multiple Src family kinases in sea urchin eggs and their function in Ca2+ release at fertilization

Egg activation at fertilization in deuterostomes requires a rise in intracellular Ca(2+), which is released from the egg's endoplasmic reticulum. In sea urchins, a Src Family Kinase (SpSFK1) is necessary for the PLCgamma-mediated signaling event that initiates this Ca(2+) release (Giusti, A.F., O'Neill, F.J., Yamasu, K., Foltz, K.R. and Jaffe, L.A., 2003. Function of a sea urchin egg Src family kinase in initiating Ca2+ release at fertilization. Dev. Biol. 256, 367-378.). Annotation of the Strongylocentrotus purpuratus genome sequence led to the identification of additional, predicted SFKs (Bradham, C.A., Foltz, D.R., Beane, W.S., Amone, M.I., Rizzo, F., Coffman, J.A., Mushegian, A., Goel, M., Morales, J., Geneviere, A.M., Lapraz, F., Robertson, A.J., Kelkar, H., Loza-Coll, M., Townley, I.K., Raisch, M., Roux, M.M., Lepage, T., Gache, C., McClay, D.R., Manning, G., 2006. The sea urchin kinome: a first look. Dev. Biol. 300, 180-193.; Roux, M.M., Townley, I.K., Raisch, M., Reade, A., Bradham, C., Humphreys, G., Gunaratne, H.J., Killian, C.E., Moy, G., Su, Y.H., Ettensohn, C.A., Wilt, F., Vacquier, V.D., Burke, R.D., Wessel, G. and Foltz, K.R., 2006. A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation. Dev. Biol. 300, 416-433.). Here, we describe the cloning and characterization of these 4 additional SFKs and test their function during the initial Ca(2+) release at fertilization using the dominant-interfering microinjection method coupled with Ca(2+) recording. While two of the new SFKs (SpFrk and SpSFK3) are necessary for Ca(2+) release, SpSFK5 appears dispensable for early egg to embryo transition events. Interestingly, SpSFK7 may be involved in preventing precocious release of Ca(2+). Binding studies indicate that only SpSFK1 is capable of direct interaction with PLCgamma. Immunolocalization studies suggest that one or more SpSFK and PLCgamma are localized to the egg cortex and at the site of sperm-egg interaction. Collectively, these data indicate that more than one SFK is involved in the Ca(2+) release pathway at fertilization.

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.

Distinct roles for multiple Src family kinases at fertilization

Egg activation at fertilization requires the release of Ca2+ from the endoplasmic reticulum of the egg. Recent evidence indicates that Src family kinases (SFKs) function in the signaling pathway that initiates this Ca2+ release in the eggs of many deuterostomes. We have identified three SFKs expressed in starfish (Asterina miniata) eggs, designated AmSFK1, AmSFK2 and AmSFK3. Antibodies made against the unique domains of each AmSFK protein revealed that all three are expressed in eggs and localized primarily to the membrane fraction. Both AmSFK1 and AmSFK3 (but not AmSFK2) are necessary for egg activation, as determined by injection of starfish oocytes with dominant-interfering Src homology 2 (SH2) domains, which specifically delay and reduce the initial release of Ca2+ at fertilization. AmSFK3 exhibits a very rapid and transient kinase activity in response to fertilization, peaking at 30 seconds post sperm addition. AmSFK1 kinase activity also increases transiently at fertilization, but peaks later, at 2 minutes. These results indicate that there are multiple SFKs present in starfish eggs with distinct, perhaps sequential, signaling roles.

Signaling through protein kinases during egg activation

Following penetration of the zona pellucida, the mammalian spermatozoon binds and fuses with the egg plasma membrane, thereby fertilizing the egg that is still arrested at the second metaphase. Fertilization initiates in the egg a sequence of events referred to as 'egg activation'. An initial increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) appears to be the very early cellular event observed which leads to the cortical granules exocytosis and resumption of meiosis. Various treatments can induce parthenogenetic activation mimicking at least part of the fertilization events. Similar to somatic cells, studies in mammalian eggs suggest that signal transduction pathways mediate egg activation. The initial increase in [Ca(2+)](i) appears to be critical for egg activation. However, other messengers such as protein kinase C (PKC) and protein tyrosine kinases (PTKs), were suggested as possible inducers of some aspects of egg activation. In the present work, studies concerning the involvement of protein kinases during egg activation in our laboratory and in others are summarized.

The Tec family of cytoplasmic tyrosine kinases: mammalian Btk, Bmx, Itk, Tec, Txk and homologs in other species

Cytoplasmic protein-tyrosine kinases (PTKs) are enzymes involved in transducing a vast number of signals in metazoans. The importance of the Tec family of kinases was immediately recognized when, in 1993, mutations in the gene encoding Bruton's tyrosine kinase (Btk) were reported to cause the human disease X-linked agammaglobulinemia (XLA). Since then, additional kinases belonging to this family have been isolated, and the availability of full genome sequences allows identification of all members in selected species enabling phylogenetic considerations. Tec kinases are endowed with Pleckstrin homology (PH) and Tec homology (TH) domains and are involved in diverse biological processes related to the control of survival and differentiation fate. Membrane translocation resulting in the activation of Tec kinases with subsequent Ca2+ release seems to be a general feature. However, nuclear translocation may also be of importance. The purpose of this essay is to characterize members of the Tec family and discuss their involvement in signaling. The three-dimensional structure, expression pattern and evolutionary aspects will also be considered.

The protein tyrosine kinase family of the human genome

As the sequencing of the human genome is completed by the Human Genome Project, the analysis of this rich source of information will illuminate many areas in medicine and biology. The protein tyrosine kinases are a large multigene family with particular relevance to many human diseases, including cancer. A search of the human genome for tyrosine kinase coding elements identified several novel genes and enabled the creation of a nonredundant catalog of tyrosine kinase genes. Ninety unique kinase genes can be identified in the human genome, along with five pseudogenes. Of the 90 tyrosine kinases, 58 are receptor type, distributed into 20 subfamilies. The 32 nonreceptor tyrosine kinases can be placed in 10 subfamilies. Additionally, mouse orthologs can be identified for nearly all the human tyrosine kinases. The completion of the human tyrosine kinase family tree provides a framework for further advances in biomedical science.

Role of the Fyn kinase in calcium release during fertilization of the sea urchin egg

Protein tyrosine kinase activity has been implicated as part of the signaling mechanism leading to the sperm-induced calcium transient following fertilization. In the present study, we have tested the role of the Fyn kinase in triggering the calcium transient by microinjecting domain-specific fusion proteins encoding regions of Fyn sequence as inhibitors of Fyn function in vivo. A fusion protein encoding the SH2 domain of Fyn caused an increase in the latent period between sperm-egg fusion and the beginning of the calcium transient and reduced the amplitude of the calcium signal. A fusion protein encoding the U + SH3 domains also caused a small increase in the latent period. Microscopic examination revealed that a large percentage of eggs injected with the U+SH3 or SH2 domains became polyspermic as a result of the delayed block to polyspermy. Affinity experiments demonstrated that the U+SH3 and SH2 domains of Fyn were capable of forming a stable complex with phospholipase Cgamma from the sea urchin egg. The results suggest that the Fyn kinase participates in the signaling events leading up to the calcium transient and may directly regulate phospholipase Cgamma activity at fertilization.

Fertilization signalling and protein-tyrosine kinases

Fertilization is initiated by species-specific gamete cell recognition, i.e. sperm-egg interaction, followed by a rapid and sustained activation of multiple cellular and biochemical events, collectively called 'egg activation', which is indispensable for successful formation of zygotic nucleus and later embryogenesis. It is well known that sperm-induced egg activation is mediated by a transient release of calcium ions that originates from the sperm entry point and propagates through the entire egg cytoplasm. It is unclear, however, what kind of upstream events prelude to the calcium transient after sperm-egg interaction. Recently, much attention has been paid to the role of protein-tyrosine phosphorylation in egg activation process by a number of studies on some well-established model organisms. These includes marine invertebrates, frogs, and mammals. In this review, we will summarize the recent findings that begin to uncover a 'missing link' between sperm-egg interaction and egg activation with emphasis on the role of egg protein-tyrosine kinases (PTKs) in Xenopus egg fertilization.

Evidence that Src-type tyrosine kinase activity is necessary for initiation of calcium release at fertilization in sea urchin eggs

The initiation of Ca(2+) release from internal stores in the egg is a hallmark of egg activation. In sea urchins, PLCgamma activity is necessary for the production of IP(3), which leads to the initial rise in Ca(2+). To examine the possible function of a tyrosine kinase in activating PLCgamma at fertilization, sea urchin eggs were treated with the specific Src kinase inhibitor PP1 or microinjected with recombinant Src-family SH2-domain proteins, which act as dominant interfering inhibitors of Src-family kinase function. Both modes of inhibiting Src-family kinases resulted in a specific and dose-dependent delay in the onset of Ca(2+) release from the endoplasmic reticulum at fertilization. The rise in cytoplasmic pH at fertilization also was inhibited by microinjection of Src-family SH2-domain proteins. Further, an antibody directed against Src-type kinases recognized a protein of ca. M(r) 57K that was enriched in the membrane fraction of eggs. The kinase activity of this protein was stimulated rapidly and transiently at fertilization, as measured by autophosphorylation and by phosphorylation of an exogenous substrate. Together, these data indicate that a Src-type tyrosine kinase is necessary for the initiation of Ca(2+) release from the egg ER at fertilization and identify a Src-type p57 protein as a candidate in the signaling pathway leading to this Ca(2+) release.

Appearance and disappearance of Syk family protein-tyrosine kinase genes during metazoan evolution

Syk family protein-tyrosine kinases are essential components of immunoreceptor signaling in mammalian lymphocytes. The absence of Syk genes from the Caenorhabditis elegans genome suggests that this kinase family is of recent evolutionary origin. Surprisingly, we have found that Hydra vulgaris, a member of the early diverging animal phylum Cnidaria, contains a gene encoding a Syk kinase. Phylogenetic analysis indicates that a single Syk family gene was present in animals prior to the gene duplication that gave rise to Syk and ZAP-70, the two mammalian Syk family genes. C. elegans also lacks a Shark protein-tyrosine kinase gene, which we show is a member of a sister group to the Syk family. We conclude that both Syk and Shark genes were lost from the genome of an ancestor of C. elegans. This natural gene knockout result indicates that neither Syk nor Shark kinases are essential for processes held in common between the nematode and other metazoans. The Hydra Syk gene is expressed in epithelial cells, a site consistent with a role for Hydra Syk in recognition of foreign cells.

Expression of a src-type protein tyrosine kinase gene, AcSrc1, in the sea urchin embryo

By screening a cDNA library and 3'-rapid amplification of cDNA ends, the cDNA for a non-receptor type protein tyrosine kinase from the sea urchin Anthocidaris crassispina was analyzed. The deduced protein (AcSrc1) with the highest identity of about 60% to mammalian Src family kinases shows the characteristic features of the Src family. AcSrc1 mRNA is maternally expressed in unfertilized eggs, while zygotic expression is first detected in blastulae and continues through the pluteus stage. Zygotic mRNA expression, visualized by in situ hybridization, is detected specifically in archenteron at the gastrula stage, while it is restricted in plutei to the midgut and hindgut, suggesting specific roles for AcSrcl in the formation and/or functions of the digestive tract. Meanwhile, western blot analysis has shown that the AcSrc1 protein is constantly expressed throughout embryogenesis. By immunostaining, it was found that the protein (distributed evenly in the cytoplasm of unfertilized eggs) is translocated to the membrane after fertilization. All through the following development, AcSrcl was localized to the peripheries of different embryonic cells, although at a relatively low level of localization at the boundaries between adjacent cells.