Responsive instruction--or instruction that foregrounds and takes up the disciplinary substance of student thinking--is both a hallmark of recent STEM education reforms and challenging to enact. This kind of instruction may be especially challenging in instructional contexts that mandate or rely on curriculum with set, structured learning trajectories for students. In this paper, we propose a new component of curricular knowledge--in particular, an understanding of the purposes of questions or sequences of questions in research-based instructional materials. We show that this kind of curricular knowledge supported the enactment of responsive instructional practices among one cohort of 14 undergraduate physics Learning Assistants, illustrating the relationship between curricular knowledge and responsive instruction. We document some of the key phases in and tools that supported the development of this cohort's curricular knowledge, lending insight into how teacher educators might support the development of curricular knowledge in their own contexts.