No abstract

The single-transmembrane receptor SorLA/LR11 contains binding domains typical for lipoprotein receptors and a VPS10 domain, which binds the neuropeptide head-activator. This undecapeptide enhances proliferation of neuronal precursor cells in a SorLA-dependent manner. Using specific inhibitors we found previously that head activator activates shedding of SorLA by the metalloprotease TACE close to the transmembrane domain releasing the large extracellular part of the receptor. Here we show that the remaining COOH-terminal membrane fragment of SorLA is processed by ␥-secretase. Inhibition of ␥-secretase by specific inhibitors or overexpression of dominant negative presenilin mutants and knock out of the presenilin genes led to accumulation of the SorLA membrane fragment and also of full-length SorLA in the membrane. In an in vitro assay we observed the ␥-secretase-dependent release of the two soluble cleavage products, the SorLA cytoplasmic domain and the SorLA ␤-peptide. These processing steps are reminiscent of a novel signaling pathway that has been described for the notch receptor. Here, the notch cytoplasmic domain is released into the cytoplasm by the ␥-secretase and migrates to the nucleus where it acts as a transcriptional regulator. In parallel we found that a fusion protein of the released cytoplasmic tail of SorLA with EGFP located to the nucleus only if the nuclear localization signal of SorLA was intact. In a reporter gene assay the cytoplasmic domain of SorLA acted as a transcriptional activator indicating that SorLA might directly regulate transcription after activation by ␥-secretase.

Hu‐K4 is a human protein homologous to the K4L protein of vaccinia virus. Due to the presence of two HKD motifs, Hu‐K4 was assigned to the family of Phospholipase D proteins although so far no catalytic activity has been shown. The Hu‐K4 mRNA is found in many human organs with highest expression levels in the central nervous system. We extended the ORF of Hu‐K4 to the 5′ direction. As a consequence the protein is 53 amino acids larger than originally predicted, now harbouring a putative transmembrane domain. The exon/intron structure of the Hu‐K4 gene reveals extensive alternative splicing in the 5′ untranslated region. Due to the absence of G/C‐rich regions and upstream ATG codons, the mRNA isoform in brain may be translated with higher efficacy leading to a high Hu‐K4 protein concentration in this tissue. Using a specific antiserum produced against Hu‐K4 we found that Hu‐K4 is a membrane‐bound protein colocalizing with protein disulfide isomerase, a marker of the endoplasmic reticulum. Glycosylation of Hu‐K4 as shown by treatment with peptide N‐glycosidase F or tunicamycin indicates that Hu‐K4 has a type 2 transmembrane topology.