Parasitic cysteine proteases are involved in parasite stage transition, invasion of host tissues, nutrient uptake, and immune evasion. The cysteine protease cathepsin F is the most abundant protein produced by fourth-stage larvae (L4) of the nematode Teladorsagia circumcincta, while its transcript is only detectable in L4 and adults. T. circumcincta cathepsin F is a recently evolved cysteine protease that does not fall clearly into either of the cathepsin L or F subfamilies. This protein exhibits characteristics of both cathepsins F and L, and its phylogenetic relationship to its closest homologs is distant, including proteins of closely related nematodes of the same subfamily.

寄生的半胱氨酸蛋白酶参与了寄生虫的阶段过渡，宿主组织的入侵，营养的吸收和免疫逃逸。半胱氨酸蛋白酶组织蛋白酶F是线虫Teladorsagia circumcincta的第四阶段幼虫（L4）产生的最丰富的蛋白质，而其转录本仅在L4和成年人中可检测到。T.circumcincta组织蛋白酶F是一种新近发展的半胱氨酸蛋白酶，不能清楚地落入组织蛋白酶L或F亚家族中。该蛋白同时具有组织蛋白酶F和L的特性，并且与其最接近的同源物的系统发生关系远，包括同一亚科的近缘线虫的蛋白。