Extracellular matrix protein 1 (ECM1), a widely expressed glycoprotein, has been shown to harbor mutations in lipoid proteinosis (LP), an autosomal recessive disorder characterized by profound alterations in the extracellular matrix of connective tissue. The biological function of ECM1 and its role in the pathomechanisms of LP are unknown. Fibulins comprise a family of extracellular matrix components, and the prototype of this family, fibulin-1, is expressed in various connective tissues and plays a role in developmental and pathologic processes. In this study, we demonstrate that ECM1, and specifically the second tandem repeat domain which is alternatively spliced, interacts with the C-terminal segments of fibulins I C and I D splice variants which differ in their C-terminal domain III. The interactions were detected by yeast two-hybrid genetic system and confirmed by co-immunoprecipitations. Kinetics of the binding between ECM1 and fibulin-ID, measured by biosensor assay, revealed a K-d of 5.71 x 10(-8) M, indicating a strong protein-protein interaction. Since distinct splice variants of ECM1 and fibulin-1 have been shown to be co-expressed in tissues affected in LP, we propose that altered ECM1/fibulin-1 interactions may play a role in the pathogenesis of this disease as well as in a number of processes involving the extracellular matrix of connective tissues. (c) 2005 Elsevier Inc. All rights reserved.