Luman is a human basic leucine zipper transcription factor that, like the herpes simplex virus transcription factor VP16, requires the host cell factor, HCF, for activity. Although both HCF and Luman have been implicated in cell growth, their biological roles have not been clearly defined. Luman conforms to a type II membrane-associated glycoprotein with its carboxyl terminus embedded in cellular membranes and its amino terminus, which contains all its identified functional domains, in the cytoplasm. Here we show that Luman is processed by regulated intramembrane proteolysis (RIP). The site 1 protease (S1P), a Golgi apparatusresident enzyme responsible for catalyzing the first step in the RIP pathway of the sterol regulatory element binding proteins (SREBPs) and ATF6, may also be involved in the processing of Luman. Thus, processing of Luman was highly stimulated by brefeldin A, a compound that causes the reflux of Golgi apparatus enzymes to the endoplasmic reticulum (ER). In addition, coexpression of Luman with S1P containing a KDEL ER retrieval signal resulted in virtually quantitative cleavage of Luman in the absence of any treatment. Finally, Luman contains a sequence, RQLR, immediately downstream from the transmembrane domain which bears similarity to the consensus S1P cleavage site identified by others. Substitution of arginine residues within this motif abolished S1P cleavage, providing robust evidence that S1P is involved in Luman processing. We observed that following S1P cleavage, the majority of the cleaved Luman was retained in cytoplasmic membranes, indicating that an additional step or enzymes yet to be identified are involved in complete cleavage and release to yield the product which ultimately enters the nuclei of cells.Luman (also known as LZIP and CREB3) is a basic leucine zipper transcription factor of the CREB/ATF gene family. It possesses a potent N-terminal acidic activation domain and a basic-leucine zipper motif (bZIP) (15,(23)(24)(25)(26). The primary structure of Luman appears to be strongly conserved, and Luman homologues in mice (LZIP [8]), cattle (our unpublished results), and fruit flies (dCREB-A/BBF-2 [1, 37]) have been identified. We and others (15, 24) originally identified Luman when screening for cellular ligands of the human host cell factor (HCF, also known as C1 factor), a protein required by the herpes simplex virus (HSV) transactivator VP16. Luman interacts with HCF through the tetrapeptide DHTY (15,24,25), which is homologous to the EHAY HCF binding sequence of VP16. This motif, as (D/E)HXY, is conserved in the VP16 homologues of other alphaherpesviruses as well as in the homologues of Luman in mice, cattle, and fruit flies. Luman can bind and activate genes containing cyclic AMP response elements (CREs), although its natural target has not been identified. Similarly, although Luman has been implicated in the regulation of cell growth (18), its biological role in this process has not been clearly defined. Luman mRNA is present in a wide range of adult and fetal tissues (...

Regulated intramembrane proteolysis of the factors SREBP and ATF6 represents a central control mechanism in sterol homeostasis and stress response within the endoplasmic reticulum. Here, we compare localization of ATF6-related bZip factors CREB4, CREB-H, Luman, and OASIS. These factors contain the defining features of a bZip domain, a predicted transmembrane domain and an adjacent cleavage site for the Golgi protease S1P, with conserved features which indicate that it represents a specific subclass of S1P sites. Each factor localizes to the endoplasmic reticulum (ER), but a population of CREB4 was also observed in the Golgi. Deletion of the transmembrane domain in CREB4 resulted in efficient nuclear accumulation. An N-terminal variant of CREB4 containing the BZIp domain potently activated expression from a target gene containing ATF6 binding sites and from the promoter for the ER chaperone GRP78/BIP. CREB4 was cleaved in a site-specific manner in response to brefeldin A disruption of the Golgi or by coexpression with S1P but only after deletion or substitution of its C-terminal lumenal domain. Thus, S1P cleavage of CREB4 may be suppressed by a determinant in the C-terminal region. Dithiothreitol induced more complete transport of CREB4 to the Golgi, but not cleavage. Together, the data identify at least one additional bZip factor whose localization responds to ER stress, and we propose a model based on these results that indicates additional levels of control of this novel class of transcription factors.