Biophysical and biological approaches come together to reveal the organisation of a supramolecular structure: the casein micelle
Résumé
Caseins, a family of acidic phosphoproteins, interact with colloidal calcium phosphate and
assembly to organize into a supramolecular structure: the casein micelle. Despite its nutritional importance and functional values, the intrinsic organisation of this complex is not elucidated.
Information about casein micelle structure has been obtained essentially from physico-chemical and biophysical experiments, including light, neutron, and X-ray scattering, and electron microscopy [1].
From these data, several models of the casein micelle have emerged but many questions remain about the specific arrangement of the caseins, the existence of not of privileged associations, the
interactions between caseins and the mineral fraction and the role of individual component in the aggregation process.
To obtain new insights into the intrinsic organisation of the casein micelle, we have
developed an original approach combining biophysical and biological methods. By using X-ray scattering and cryomicroscopy, we have acquired morphological knowledge on the calcium phosphate nanoclusters [2]. Recently, we have exploited the spatio-temporal dimension of casein micelle
biogenesis while studying the dynamic of elaboration of the micelle within the secretory pathway of mammary epithelial cell. We believe that to obtain new insights into the cellular mechanisms of casein
association and secretion, it is important to better understand the aggregation process and the functionalities of the casein micelle.
With this aim, we investigated primary steps of casein interaction in the rough endoplasmic reticulum (ER). A rough ER microsomal fraction was purified from rat mammary gland cells and ER membranes were permeabilized with saponin, in conservative or non-conservative conditions for casein micelle, and centrifuged to separate soluble from insoluble (aggregated or membrane associated)proteins. We concluded that immature b-casein contained in rough microsomes was at most weakly associated with primary casein aggregates whereas a large fraction of immature aS1-casein remained associated to permeabilized microsomes. Also, extraction of membranes in various physico-chemical conditions demonstrated that a substantial fraction of aS1-casein exists as a membrane-associated form within the rough ER and in more distal compartments of the secretory pathway [3].
Since aS1-casein, is essential for the export of other caseins from the ER to the Golgi apparatus [4], our data strongly suggest that the membranous form of aS1- casein might play a key role in casein micelle formation and/or in its transport within the secretory pathway and casein secretion.
Future studies concerning the elucidation of casein micelle structure will be at the interface of biological and biophysical methods.