In this study, we investigated the pedagogical content knowledge (PCK) that physics graduate teaching assistants (TAs) developed in the context of teaching a new introductory physics curriculum, "Matter and Interactions" ("M&I"). "M&I" is an innovative introductory physics course that emphasizes a unified framework for understanding the world and presents physics through a few fundamental principles rather than exposing students to concepts through a series of derived equations. Through a qualitative, multiple case study research design, data were collected from multiple sources: non-participant observations, digitally recorded video, semi-structured interviews, TAs' written reflections, and researchers' field notes. The TAs' PCK included three components: (a) knowledge of "M&I" curriculum goals, (b) knowledge of instructional strategies appropriate to the "M&I" course, and (c) knowledge of students' learning. This study shows the complexity of adopting curriculum reforms and the necessity to support the faculty's and TAs' knowledge development when a novel science curriculum is adopted.