The recognized structural proteins of the enamel matrix are amelogenin, ameloblastin, and enamelin. While a large volume of data exists showing that amelogenin self-assembles into multimeric units referred to as nanospheres, other reports of enamel matrix protein-protein interactions are scant. We believe that each of these enamel matrix proteins must interact with other organic components of ameloblasts and the enamel matrix. Likely protein partners would include integral membrane proteins and additional secreted proteins.Introduction: The purpose of this study was to identify and catalog additional proteins that play a significant role in enamel formation. Materials and Methods: We used the yeast two-hybrid assay to identify protein partners for amelogenin, ameloblastin, and enamelin. Once identified, RT-PCR was used to assess gene transcription of these newly identified and potential "enamel" proteins in ameloblast-like LS8 cells. Results: In the context of this yeast assay, we identified a number of secreted proteins and integral membrane proteins that interact with amelogenin, ameloblastin, and enamelin. Additionally, proteins whose functions range from the inhibition of soft tissue mineralization, calcium ion transport, and phosphorylation events have been identified as protein partners to these enamel matrix proteins. For each protein identified using this screening strategy, future studies are planned to confirm this physiological relationship to biomineralization in vivo. Conclusion: Identifying integral membrane proteins of the secretory surface of ameloblast cells (Tomes' processes) and additional enamel matrix proteins, based on their abilities to interact with the most abundant enamel matrix proteins, will better define the molecular mechanisms of enamel formation at its most rudimentary level.

Yeast proteins Ntr1, Ntr2 and Prp43 function in spliceosome disassembly. An Ntr1-Ntr2 protein complex recruits Prp43 to allow the removal of the lariat-intron in latestage RNA splicing activity. Based on amino-acid sequence similarities across species, TFIP11 and mDEAH9/Dhx15 have been identified as homologues of yeast Ntr1 and Prp43, respectively. The N-terminal region of TFIP11 contains a G-patch, which is a highly conserved domain of many RNA-processing proteins. TFIP11 displays a unique and characteristic subnuclear localization pattern, in close proximity to SC35 nuclear speckles. Transfected GFP-tagged mDEAH9 displays an evenly distributed nuclear localization and is excluded from the nucleoli; however when TFIP11 and mDEAH9 are co-transfected, both proteins colocalize to distinct nuclear speckles. These data show that TFIP11 recruits mDEAH9 suggesting that these two proteins have similar biological activities to their yeast counterparts.

Tuftelin-interacting protein 11 (TFIP11) is a protein component of the spliceosome complex that promotes the release of the lariat-intron during late-stage splicing through a direct recruitment and interaction with DHX15/PRP43. Expression of TFIP11 is essential for cell and organismal survival. TFIP11 contains a G-patch domain, a signature motif of RNA-processing proteins that is responsible for TFIP11-DHX15 interactions. No other functional domains within TFIP11 have been described. TFIP11 is localized to distinct speckled regions within the cell nucleus, although excluded from the nucleolus. In this study sequential C-terminal deletions and mutational analyses have identified two novel protein elements in mouse TFIP11. The first domain covers amino acids 701-706 (VKDKFN) and is an atypical nuclear localization signal (NLS). The second domain is contained within amino acids 711-735 and defines TFIP11's distinct speckled nuclear localization. The identification of a novel TFIP11 nuclear speckle targeting sequence (TFIP11-STS) suggests that this domain directly interacts with additional spliceosomal components. These data help define the mechanism of nuclear/ nuclear speckle localization of the splicing factor TFIP11, with implications for it's function.

The established structural proteins of the enamel matrix are amelogenin, ameloblastin, and enamelin. Historically, tuftelin and tuftelin-interacting protein 11 (TFIP11) have also been discussed as possible enamel proteins. Protein complexes are achieved by protein-protein interactions, and it is protein complexes that control biomineralization. The purpose of our recent studies was to catalog protein partners for these proteins that are, or have been, implicated in tooth formation. We used the sensitive yeast two-hybrid assay to identify proteins that interact directly with amelogenin, ameloblastin, enamelin, the leucine-rich amelogenin peptide (LRAP) and TFIP11. In this manuscript we refer to, or document, potential protein partners for the proteins listed above. The yeast two-hybrid assay may ultimately prove to be a valuable proteomics methodology for using to decipher molecular events that ultimately result in enamel biomineralization.

Previous studies using the yeast two-hybrid assay (Y2H) have identified cyclin L1 (CCNL1) and Ewing sarcoma breakpoint region 1 protein (EWSR1) as being interacting partners of tuftelin-interacting protein 11 (TFIP11). All three proteins are functionally related to the spliceosome and involved in pre-mRNA splicing activities. The spliceosome is a dynamic ribonucleoprotein complex responsible for pre-mRNA splicing of intronic regions, and is composed of five small nuclear RNAs (snRNAs) and ~140 proteins. TFIP11 appears to play a role in spliceosome disassembly allowing for the release of the bound lariat-intron. The roles of CCNL1 and EWSR1 in the spliceosome are poorly understood. Using fluorescently-tagged proteins and confocal microscopy we show that TFIP11, CCNL1 and EWSR1 frequently co-localize to speckled nuclear domains. These data would suggest that all three proteins participate in a common cellular activity related to RNA splicing events.