Required for the correct formation of lens intermediate filaments as part of a complex composed of BFSP1, BFSP2 and CRYAA (PubMed:15037121, PubMed:27559293). Plays a role in maintenance of retinal lens optical clarity (PubMed:12454043). Part of a complex required for lens intermediate filament formation composed of BFSP1, BFSP2 and CRYAA (By similarity). Found in a complex composed of PPL (via C-terminal linker domain), BFSP1 and BFSP2 in the retinal lens (PubMed:19029034). Within the complex interacts with PPL (via C-terminal linker domain) and with BFSP1 (PubMed:19029034). Identified in a complex that contains VIM, EZR, AHNAK, BFSP1, BFSP2, ANK2, PLEC, PRX and spectrin (PubMed:21745462). Interacts with LGSN (PubMed:18178558). Interacts with VIM (PubMed:19029034). Expressed primarily at the plasma membrane in peripheral lens fiber cells, however also localizes to the cytoplasm in mature lens fiber cells. Detected in retina lens fiber cells (at protein level) (PubMed:7679620, PubMed:12454043, PubMed:15037121, PubMed:14985306, PubMed:19029034, PubMed:21745462, PubMed:27559293). Also expressed in the lens epithelium, abundantly expressed in the anterior and anterolateral epithelium, less frequently expressed nearer the lens coronal equator (at protein level) (PubMed:27559293). Expressed in the retinal lens fiber cells from postnatal day 28 (P28) to P45 (PubMed:27559293). Expressed in retinal lens beaded filament structures from P37 onwards (PubMed:27559293). In strains 101, 129/SvJ and CBA, a polymorphism deletes the acceptor site of exon 2 which causes exon 1 to be spliced to exon 3 and generates a frameshift and premature stop codon (PubMed:12573667, PubMed:15037121). The polymorphism leads to retinal lens fiber cell nuclear opacity beginning at 1 month of age, opacity becomes more pronounced with age (PubMed:14985306). Reduced intermediate filaments and loss of the association of the lens fiber cell cytoskeleton with the plasma membrane (PubMed:15037121, PubMed:14985306). No overall change in lens fiber cell organization, regularity of hexagonal profiles, or positioning of cell nuclei (PubMed:12454043). Opacification of the retinal lens is evident at 1 month of age, with progressive loss of clarity to 10 months of age (PubMed:12454043). Reduced abundance and loss of distinction of intermediate filaments in retinal lens fiber cells (PubMed:15037121). Decreased protein abundance of BFSP1 in the retinal lens (PubMed:12454043). Complete loss of beaded filament structures in lens epithelial cells (PubMed:27559293). BFSP2 and VIM double knockout mice show a complete loss of the cytoplasmic cytoskeleton in retinal lens fiber cells (PubMed:15037121). Belongs to the intermediate filament family.