IDENTIFICATION AND CHARACTERIZATION OF A NEW LATENT TRANSFORMING GROWTH FACTOR-BETA-BINDING PROTEIN, LTBP-4

Transforming growth factor beta (TGF-beta s) are secreted by most cell types as latent high molecular weight complexes consisting of TGF-bet a and its latency associated peptide (LAP) propeptide dimers, covalent ly linked to latent TGF-beta-binding proteins (LTBPs), Currently, thre e different LTBPs are known (LTBPs 1, 2, and 3), all with highly simil ar protein domain structure consisting of epidermal growth factor-like and 8-Cys repeats. The 3rd 8-Cys repeat of LTBP-1 mediates its associ ation with TGF-beta 1 . LAP. By using an expressed sequence tag homolo gous to the 3rd 8-Cys repeat of human LTBP-1 as a probe, a novel cDNA similar to known LTBPs was cloned from human heart cDNA Library. This cDNA was named LTBP-4 and found to exist in at least four different fo rms, generated by alternative splicing at the amino terminus and at th e central epidermal growth factor repeat domain. One of the alternativ e amino-terminal forms contained an RGD sequence, indicating possible cell-surface interactions with integrins. LTBP-4 gene was localized to chromosomal position 19q13.1-19q13.2. The major LTBP-4 mRNA form is a bout 5.1 kilobase pairs in size and is predominantly expressed in the heart, aorta, uterus, and small intestine. Immunoblotting analysis ind icated that LTBP-4 was secreted from cultured human lung fibroblasts b oth in a free form and in a disulfide bound complex with a TGF-beta.LA P-like protein. Both LTBP-4 forms were also found to be deposited in t he extracellular matrix. The matrix-associated LTBP-4 was susceptible to proteolytic release with plasmin. LTBP-4 is a new member of the gro wing LTBP-fibrillin family of proteins and offers an alternative means for the secretion and targeted matrix deposition of TGF-beta s or rel ated proteins.