Component of the gypsy chromatin insulator complex which is required for the function of the gypsy chromatin insulator and other endogenous chromatin insulators. Chromatin insulators are regulatory elements which establish independent domains of transcriptional activity within eukaryotic genomes. Insulators have two defining properties; they can block the communication between an enhancer and a promoter when placed between them and can also buffer transgenes from position effect variegation (PEV). Insulators are proposed to structure the chromatin fiber into independent domains of differing transcriptional potential by promoting the formation of distinct chromatin loops. This chromatin looping may involve the formation of insulator bodies, where homotypic interactions between individual subunits of the insulator complex could promote the clustering of widely spaced insulators at the nuclear periphery. Within the gypsy insulator complex, this protein binds specifically to a region of the gypsy element located 3' of the 5' long terminal repeat (LTR), and may also mediate interaction with other endogenous insulators at sites distinct from those recognized by Cp190. Cooperates with pita and cliff to recruit Cp190 and regulate insulator function at the front-ultraabdominal (Fub) boundary (PubMed:38769058).

(UniProt, P08970)

In homozygous condition (e.g., su(Hw)2/su(Hw)2 or su(Hw)2/su(Hw)7) suppresses certain spontaneous alleles that contain gypsy inserts at a number of different loci, e.g., y2, Hw1, sc1, scD2, dm1, ct6, ctK, lz1, f1, f5, fK, B, Bxd2, h1, bx3, bx34e, bxd, bxd51j, bxd55i, bxdK, and ci1; alleles without gypsy inserts are not suppressed (see Lewis, 1949). Apparent exceptions to the above generalizations are scD1, which is suppressed and reportedly X ray induced and rsP1 and rsP2, both of which show temperature-sensitive suppression but no evidence of gypsy insertion. With the exception of lz37, which is enhanced by su(Hw), alleles not known, or known not, to contain gypsy, but which are modified by other suppressors are unaffected by su(Hw). Two alleles, ctK and fK, exhibit suppression in heterozygotes for either su(Hw) or a deficiency for su(Hw). su(Hw) causes accumulation of f transcript in flies carrying f alleles that ordinarily display low levels to return to wild-type levels (Parkhurst and Corces, 1985, Cell 41: 429-37); judging from results with y2, this is caused by reduction of transcription from the associated gypsy element (Parkhurst and Corces, 1986, Mol. Cell. Biol. 6: 47-53). Females homozygous for su(Hw)1, su(Hw)2, su(Hw)3, and su(Hw)4 are sterile; females homozygous for su(Hw)8 and transheterozygotes of su(Hw)7 with other allelles are fertile; su(Hw)7/su(Hw)2 are fertile and, in fact, suppress the female sterility of dm and lz (Grell). In sterile combinations, vitellogenesis inhibited leading to smaller-than-normal cysts surrounded by multiple layers of follicle cells; nurse cell chromosomes remain condensed until stage 9, after which egg chambers degenerate; ovarian phenotype autonomous in transplants (Klug et al., 1968, 1970). Reduced viability attributed to some alleles apparently caused by extraneous genes, since those alleles are perfectly viable in combination with su(Hw) deficiencies.