With the emerging of mobile communication technologies, we are entering the fifth generation mobile communication system (5G) era. Various application scenarios will arise in the 5G era to meet the different service requirements. Different 5G network slicings may deploy different public key cryptosystems. The security issues among the heterogeneous systems should be considered. In order to ensure the secure communications between 5G network slicings, in different public cryptosystems, we propose two heterogeneous signcryption schemes which can achieve mutual communications between the Public Key Infrastructure (PKI) and the CertificateLess public key Cryptography (CLC) environment. We prove that our schemes have the INDistinguishability against Adaptive Chosen Ciphertext Attack (IND-CCA2) under the Computational Diffie-Hellman Problem (CDHP) and the Existential UnForgeability against adaptive Chosen Message Attack (EUF-CMA) under the Discrete Logarithm Problem (DLP) in the random oracle model. We also set up two heterogeneous cryptosystems on Raspberry Pi to simulate the interprocess communication between different public key environments. Furthermore, we quantify and analyze the performance of each scheme. Compared with the existing schemes, our schemes have greater efficiency and security.