Role of the C-terminal domain of pulmonary surfactant protein A in binding to alveolar type II cells and regulation of phospholipid secretion

Abstract

Surfactant protein A (SP-A), with a reduced denatured molecular mass of 26-38 kDa, is characterized by a collagen-like sequence in the N-terminal half of the protein. This protein forms an oligomeric structure which is dependent upon this collagenous domain. SP-A has been demonstrated to function as an inhibitor of phospholipid secretion by primary cultures of alveolar type II cells via a cell surface receptor for the protein. However, the receptor-binding domain of SP-A has not been identified. The purpose of the present study was to investigate the role of the C-terminal domain of SP-A in binding to type II cells and regulation of phospholipid secretion. A monoclonal antibody to human SP-A, whose epitope was localized at the C-terminal domain of the protein, abolished the inhibitory activity of human SP-A on lipid secretion by type II cells, and attenuated the ability of human SP-A to compete with 125I-(rat SP-A) for receptor binding. SP-A was then digested with collagenase and the collagenase-resistant fragment (CRF), which is the C-terminal domain of SP-A (thus lacking the N-terminal domain), was isolated. Gel filtration chromatography revealed that CRF exists as a monomer in solution containing Ca2+. CRF had the ability to inhibit phospholipid secretion, although at a higher concentration than for SP-A, and was also able to compete with 125I-(rat SP-A) for binding to type II cells. A direct binding study showed that CRF bound to type II cells in a concentration-dependent manner. The present study demonstrates that the non-collagenous, C-terminal, domain of SP-A is responsible for the protein's inhibitory effect on lipid secretion and its binding to type II cells.

Secretagogues Increase the Expression of Surfactant Protein A Receptors on Lung Type II Cells

“…The capacity of SP-A to function as the modulator of surfactant secretion and turnover appears to involve the binding of SP-A to type II cells, probably via specific cell surface receptors (Murata et al, 1993;Wright et al,, 1989;Kuroki et al, 1988). Previously we have reported that secretagogues enhanced the uptake of lipid and protein components of lung surfactant by isolated perfused lungs (Fisher et al, 1991) and by type II cells (Chinoy et al, 1993).…”

“…Thus, SP-A may play a pivotal role in the regulation of surfactant turnover. Type II cells express a high affinity receptor for SP-A (Kuroki et al, 1988, Wright et al, 1989) that recognizes the carboxyl-terminal portion of the SP-A molecule (Wright et al, 1989;Murata et al, 1993). Using an anti-idiotypic antibody approach, two groups have identified type II cell membrane proteins specific for SP-A with molecular masses of either 30-kDa (Strayer et al, 1993) or 55 kDa (Stevens et al, 1995).…”

Analysis of Chimeric Proteins Identifies the Regions in the Carbohydrate Recognition Domains of Rat Lung Collectins That Are Essential for Interactions with Phospholipids, Glycolipids, and Alveolar Type II Cells

“…SP-A also interacts with alveolar type II cells (10,11). Previous studies have identified the CRDs as essential for the interactions of lung collectins with lipids and type II cells (31)(32)(33). In this study, we focused on the CRD regions of SP-A and SP-D, and progressively longer carboxyl-terminal regions of the SP-D CRD were substituted for the corresponding SP-A CRD regions.…”

“…Although lung collectins possess homologous structures and exhibit similar carbohydrate binding specificities, they show quite different properties of binding lipids and interacting with alveolar type II cells. Previous studies have suggested that the CRD of SP-A or SP-D plays a critical role in interactions with lipids and type II cells (31)(32)(33)(34). However, the regions of the CRDs that are essential for lipid binding and type II cell interactions have not yet been fully identified.…”

Mutational Analysis of Arg197 of Rat Surfactant Protein A

“…Receptor binding is required for inhibition of surfactant secretion and enhancement of phospholipid uptake activities by SP-A (18,19,32 (Fig. 2).…”