The present invention provides novel polynucleotides encoding MGAT3 polypeptides, fragments and homologues thereof. Also provided are vectors, host cells, antibodies, and recombinant and synthetic methods for producing MGAT3 polypeptides, fragments, and homologues thereof. The invention further relates to diagnostic and therapeutic methods for applying these novel MGAT3 polypeptides to the diagnosis, treatment, and/or prevention of various diseases and/or disorders related to these polypeptides, such as obesity. The invention further relates to screening methods for identifying agonists and antagonists of the polynucleotides and polypeptides of the present invention.