A Test of Current Models for the Mechanism of Milk‐Lipid Droplet Secretion. (23rd June 2013)

• Record Type: Journal Article
• Authors: Jeong, Jaekwang; Lisinski, Ivonne; Kadegowda, Anil K. G.; Shin, Hyunsu; Wooding, F. B. Peter; Daniels, Brian R.; Schaack, Jerome; Mather, Ian H.
• Is Part Of: Traffic. Volume 14:Number 9(2013:Sep.)
• Page Start: 974
• Page End: 986
• Publication Date: 2013-06-23
• Subjects: butyrophilin -- exocrine biology -- lactation -- milk‐lipid secretion -- mouse -- perilipin‐2 -- xanthine oxidoreductase; Biological transport -- Periodicals; 571.6
• DOI: 10.1111/tra.12087
• Languages: English
• ISSNs: 1398-9219

Abstract

Abstract : Molecular models for the secretion of milk‐lipid droplets are based on the assumption that the integral protein, butyrophilin (BTN), interacts with proteins on the lipid droplet surface to form an immobile secretion complex. Using a combination of morphological and biochemical approaches, we show that in mouse milk‐lipid droplets, BTN is a minor and highly mobile component on the droplet surface. A more dynamic model, in which BTN transiently interacts with xanthine oxidoreductase and other lipid‐associated proteins is proposed. Abstract : Milk lipid is secreted by a unique process, during which triacylglycerol droplets bud from mammary cells coated with an outer bilayer of apical membrane. In all current schemes, the integral protein butyrophilin 1A1 (BTN) is postulated to serve as a transmembrane scaffold, which interacts either with itself or with the peripheral proteins, xanthine oxidoreductase (XOR) and possibly perilipin‐2 (PLIN2), to form an immobile bridging complex between the droplet and apical surface. In one such scheme, BTN on the surface of cytoplasmic lipid droplets interacts directly with BTN in the apical membrane without binding to either XOR or PLIN2. We tested these models using both biochemical and morphological approaches. BTN was concentrated in the apical membrane in all species examined and contained mature N‐linked glycans. We found no evidence for the association of unprocessed BTN with intracellular lipid droplets. BTN‐enhanced green fluorescent protein was highly mobile in areas of mouse milk‐lipid droplets that had not undergone post‐secretion changes, and endogenous mouse BTN comprised only 0.5–0.7% (w/w) of the total protein, i.e. over 50‐fold less than in the milk‐lipid droplets of cow and other species. These data are incompatible with models of milk‐lipid secretion in which BTN is the major component of an immobile global adhesive complex and suggest that interactions between BTN and other proteins at the time of secretion are more transient than previously predicted. The high mobility of BTN in lipid droplets marks it as a potential mobile signaling molecule in milk .